lustration, and will briefly subjoin such observations as I have made, with a view to explain the appearances they present.

a, the Lithodomus containing one or more specimens of Modiola.

b, the opposite

side of the same specimen, but with the external shell broken away, so as to show one of the contained Modiola. c, a Lithodomus in which, from the gaping of the valves of the inclosed Modiola, three or four individuals may be distinguished.

The size of the figures is enlarged by half a diameter.

1. It will be observed in the specimen (fig. 69 a) that the outer shell is extremely different from that which it contains (see b). Now although I have repeatedly detected a similar arrangement-the outer smooth shell (Lithodomus) with its strongly-marked lines of growth containing, and the sharp, angulated, reticulated shell (Modiola?) being contained-yet I never met with an instance in which this order was reversed. This I conceive to be a particular of some importance.

2. Among the many specimens that have come under my observation, I have never seen a single instance in which the contained Modiola (?) could be distinctly shown to be a boring shell. Even when it appears to occupy a perforation by itself, the difference in size between the hole in which it is situated and itself, and sometimes other circumstances additional to this, seem to show that it is merely the inhabitant but not the fabricator of the orifice in which it has existed.

3. In cases in which there are more than one contained shell (as shown by fig. 69. c), the additional ones are, I believe, uniformly of the same species with the first-contained shell, which is constantly a Modiola and never a Lithodomus. 4. Although these shells are almost invariably found enveloping one another like a nest of pill-boxes, yet I have in one instance seen two small ones placed endwise, the one towards the other, filling up the cavity of a much larger Lithodomus.

After what I have said it is scarcely necessary to add that I consider the contained shell a true Modiola, and consequently not a boring animal:-that it occupied the cavities formed in the coral by the Lithodomi, and very frequently filled the unoccupied shells of the Lithodomi themselves.But although it might be supposed that one Modiola when

young had made its way within the half-closed valves of a Lithodomus, it is difficult to imagine that this process could go on in a second, third, or even fourth instance, since in each case the death of the previous inhabitant must have been a necessary condition; and the former occupant, which could have obtained entrance only in a very young state, must have lived long enough to fill the entire cavity with its shell. It is difficult also to account for the fact of the same species only of Modiola enveloping each other, upon the supposition of a fortuitous occupation of the empty shell by the young animal; since as there are more than one species of Modiola in the same locality, it would have been quite as easy for one of these to have made its way in as the other.

A case somewhat analogous had recently come under my observation through the kindness of a friend, in the instance of the Saxicava rugosa, in the interior of which specimens of Venerupis perforans are sometimes met with. But in this case the size of the contained shell does not at all correspond with that which contains it, and moreover the one Venerupis does not in any instance contain another.

[A series of specimens illustrative of the present communication have been kindly submitted to our examination by Mr. Jelly; and from these we selected the two of which representations are given (fig. 69). We can suggest no other explanation but the obvious one of supposing that the dead shell of the Lithodomus was occupied by a Modiola, and the Modiola itself subsequently occupied by a smaller individual of its own species; the same thing being repeated, in some instances, five or six times. The introduction however of the Modiola in the adult state would be opposed by the physical condition in which the Lithodomus is placed. Any suggestions or observations from our conchological readers, bearing upon this curious fact, would be acceptable.-ED.]

ART. VII.-An Account of the Strata of Lincoln, from a recent Survey, commencing North of the Cathedral, and descending to the bed of the River. Drawn up by MR. Wм. Bedford.1 THE strata may be comprised in twenty-six beds, which slightly vary and thin off, in some parts; but lie horizontally, from six to eighteen inches in thickness, (with the exception of the Upper Oolite), till we descend to the Ochry Ferruginous-stone beds.

1. Alluvial soil, from six to ten inches in thickness.

2. Rubbly stone;-Cardia or stone cockles are profusely distributed here.

1 Communicated by Sir Edward Ff. Bromhead, Bart.
3 P

VOL. III.-No. 35, N. s.

3. Called the Blue bed, a hard limestone, wherein spar and crystalline cockles are found.

4. Knobbly or Boss rubble;-contains casts of shells.

A layer of marle lies underneath.

5. The Shell bed;-stone cockles in great variety are found in this bed.

A layer of marle lies underneath.

6. The Blue Limestone bed;-contains the Mactra, a kind of muscle.

7. Three beds of the Grey Limestone, each bed intercepted with marle; oysters, Murex, the lobster-tailed nautilus or miller's thumb, and the Chiton, [?-Ed.] are found in these beds.

8. Three beds of fractured limestone, each bed intercepted with a layer of marle.

9. A strong limestone bed called the Roof bed, under which the ancient builders excavated or rather mined, for superior stone for building the Cathedral, which may account for the numerous caverns and subterraneous places to a great extent. A very large portion of the upper part of Lincoln, and nearly the whole of Eastgate, is thus undermined.

10. Three thin knobbly beds intercepted with marle. 11. The Oolite Freestone bed;-calc spar occurs here in

rhombic and prismatic crystals. Large Ammonites, and the Teredo or Lapis Syringoides, and fossil wood, are found in this bed.

12. The Silver bed ;-it abounds with cornbrash and Archimedes shells; it is allied to the forest-marble, and when faced, is used for chimney-pieces and for floors of passages; it decomposes oily matters, and is a durable stone for buildings in dry situations;-prismatic and rhomboid calc spar is found in this bed.

13. A bed of good building stone, superior to the silver bed, about sixteen inches in thickness;-this bed abounds in some parts with cornbrash and Archimedes shells, the same as the silver bed; in other parts it is free from cornbrash. Between the fissures in this bed, the agaric mineral occurs in delicate opaque crystals. The dagger shells, razor-sheath, and various other shells, are found in this bed.

14. Two beds of good stone, with oolite disseminated, useful for foundations and building purposes. In the first bed fossilized branches of trees sometimes occur, lying horizontally. Prismatic calc spar in bold crystals occurs in this bed.

The quarrymen in the present day do not work below these beds.1 15. The Oolite2 or Roe-stone bed is nearly two feet in thickness. Newport Arch, erected nearly 1700 years ago, and for its Roman origin an object of much interest to travellers, was built of the stone from the oolite bed. It is a hard oolite, and becomes harder by exposure to a humid atmosphere, which may account for its durability. In some parts of this stratum it is Blue-hearted. Large blocks of this oolite may be seen in the main street, a little above the Hospital gates, being the remains of the south Roman gate, long since destroyed. The Cathedral is evidently built of the stone from the silver bed-of that which underlies the silver bed-and from the beds now used for foundations and walls, with a portion of the oolite bed. John of Gaunt's house, now a modernized dwelling, and many years the residence of the late Mr. Boot, seems chiefly built of this oolite.

16. A bed of indurated clay, six inches in thickness. 17. A bed of very hard blue stone, which divides itself into two beds, by a flaw passing longitudinally through the middle.

A bed of very hard indurated clay, four inches thick, divides the above bed from

18. A thin bed of hard fine sandstone, firmly united to 19. The Grey oolite bed, which is as firmly united to 20. The White oolite bed. These three contiguous beds form indeed one massive bed, nearly four feet in thickness, equal in hardness to the oolite bed of which Newport Arch is built. About an inch of clay intervenes between this white oolite and the

21. Lower oolite bed, which is not so hard as the beds above, and which lies upon a bed of yellow ochry earth, underneath which the springs begin to appear.

3

22. 4Ochry ferruginous-stone bed;-the spring water near Monks' House flows through its fissures, and deposits the ferruginous ochre as it streams along.

23. Ferruginous gravel and sand bed, underneath which Pyrites in masses occur in some parts, just as we enter the

1 The stone-quarries are the best places for examining the strata.
2 The oolite will not burn into quicklime.

3 There are no springs in the lower part of Lincoln, the water obtained there by the sinking of wells, is the river water, which is filtered through the sand bed.

This may be seen to advantage at the north-east corner of the Monk's Leas.

I

24. Thick bed of Clunch clay; Ammonites, Nautili, and Belemnites occur in this bed.

25. Ferruginous gravel and sand bed,-intervenes between the two beds of clay, with nodules of iron Pyrites. 26. Thick bed of Blue clay-shale, an excellent clay, when ground, for tiles and floor-bricks. In this bed are three seams of rubbly ironstone-clay, which dip towards the east, from three to four inches in thickness;--the second seam is two feet below the first, and the third seam between three and four feet below the second. Fossilized oysters, muscles, and periwinkles are found in this bed. This clay bed is of great thickness, and declines with the slope of the hill; it dips beneath the sand bed of the river, and rises again as we ascend Cross o' Cliff hill. The minerals and fossils of the various beds have been carefully selected for the Museum of the Lincoln Mechanics' Institution.

ART. VIII.-On the Structure and Habits of the Physalia (of Cuvier) or Portuguese Man-of-War; Holothuria Physalis, of Linnæus.By JONATHAN COUCH, Esq., F.L.S.

I HAVE not been able to find in any book to which I have access, such an account of the Physalia as affords an insight into its manner of existence, or adequately represents its peculiarities of form or structure. The former, indeed, may be regarded as very simple, as is the case with the greater part of animals which are low in the scale of organization. But wherein they are deficient in extent of endowment, they obtain compensation in the precision of that one function with which their existence is identified; and in this respect our judgment in regard to some of the obscure or ill-understood functions of the organs of higher animals, may be informed and corrected by what is more clearly-because more singly-seen in the actions of these creatures.

In the days of Pennant the Physalia had not been recognized in the British seas. Yet it is not of rare occurrence, and sometimes appears in considerable numbers, keeping in a loose arrangement of companies, floating buoyantly on the surface, and carried wherever the wind and tide are disposed to bear them.

In the descent of the Steep Hill, the great thoroughfare of Lincoln, the clay is indurated, and cannot be made plastic. This clay-shale is from 60 to 90 feet in thickness, and must be bored through into the heart of a rocky crust lying below, before water can be obtained. Water can only be obtained above and below this indurated clay.