all are equally omnivorous. The Phalangers and Petaurists, indeed, display so decided a preference for live birds, as to make it probable that these constitute a main portion of their food in a state of nature; whilst the Opossums, as it is well known, thrive equally well upon fruit and vegetables as upon flesh, and feed indifferently upon either.

Nor are the modifications of dentition proper to the Opossums and Phalangers, respectively, so very different in reality as they appear to be at first sight. The Opossums have ten incisors in the upper and eight in the lower jaw, canines of the usual form and number, and seven molars throughout, of which four only are true molars, and have flat crowns with blunt tubercles, like the Símiæ and Lemúridæ : the Phalangers and Petaurists, on the contrary, have only six incisor teeth in the upper jaw, and two in the lower; the latter long and procumbent, and both separated from the true molars by a vacant space containing two insulated false molars, generally rudimentary, but sometimes developed to such an extent as to present the appearance and exercise the functions of real canines. In some species, these anomalous canines are in contiguity with the lateral incisors of the upper jaw, and are in all cases situated upon the suture connecting the maxillary and intermaxillary bones; in other instances, the inferior false molars, though rudimentary, are contiguous to the long procumbent incisors, and inclined in the same direction; so that these teeth may be considered, without impropriety, as lateral incisors in both the upper and lower jaws; a view which greatly diminishes the apparent dissimilarity between the dental systems of the Phalangers and Opossums. The molar teeth of these two genera are still more closely allied in form, being equally provided with flat crowns and blunt tubercles; only that those of the Phalangers approach more nearly to the molars of the Símiæ, both in form and number, than those of the Opossums. Still it is unquestionable that a considerable hiatus does exist between these two systems of dentition; and, though it might appear in some degree to be filled up by the Dasyures, yet more influential parts of their structure exclude that genus from the present group. At all events, whatever difference may exist in the organs of mastication, between the extreme genera which I have associated in this family, there is, as it has been already observed, but little difference of function, and the natural habits and appetites are nearly the same in all.

But there are other and very influential parts of the organic structure of these animals, in which the chain of affinities is less broken, and more easily recognised, than in their dental

systems. From the naked prehensile-tailed Opossums of South America, for instance, we have a gradual and uninterrupted transition, through the equally naked-tailed Coescoes (Balántia) of the Indian Isles, to the true Phalangers; and from these to the Petaurists directly on the one hand, and, by means of the Pseudocheirs, to the Koalas on the other. Here the chain is perfectly unbroken, and the affinities of the different genera too obvious to be overlooked; and this new relation, added to their common habits, appetites, and economy, the conformity of structure displayed in their organs of sense and prehension, and the marsupial character of the whole tribe, constitutes a mass of evidence in favour of the approximation of these animals which I have here made, as strong as that which supports almost any other natural family. I propose, therefore, to distinguish the present group by the name of Didelphidæ, in allusion at once to its most remarkable character, and to the principal genus of which it is composed.

(To be continued.)

ART. II. Observations on some Species of the Genus Motacilla of Linnæus. By JOHN GOULD, Esq., F.L.S., &c.

HAVING recently brought before the notice of the Zoological Society, at one of the scientific meetings, a few remarkable facts respecting the limited range of some of our native birds, and the strictness with which they are confined to certain localities, I hastily transmit a few additional observations, which, as they principally relate to one of the most elegant and familiar of the British birds, may, perhaps, be worthy of a place in the Magazine of Natural History, where they will meet the eyes of many devoted exclusively to the study of British ornithology, and, perhaps, induce a more minute investigation of the subject to which the present observations are directed.

The distinctions pointed out by me, a few years since, between the yellow wagtail of the British islands (Motacilla flava Ray) and the species commonly seen on the neighbouring continent, which I have named M. neglécta, are now, I believe, well known to most of your readers. My views respecting the separation of these species have since been confirmed by the opinions of ornithologists generally; and, although a few solitary specimens have been discovered in Britain, it must be admitted that the shores of France and Holland constitute the western boundary of the species; while, as far as my observations go, the British islands constitute as exclusively

the habitat of the M. flàva; its occurrence on any part of the continent being extremely infrequent.

While engaged upon this tribe of birds during the course of my work on the Birds of Europe, I was equally surprised to find that the sprightly and pied wagtail, so abundant in our islands at all seasons, could not be referred to any described species, and that it was equally as limited in its habitat; for, besides the British islands, Norway and Sweden are the only parts of Europe whence I have been able to procure examples identical with our bird, whose place in the temperate portions of Europe is supplied by a nearly allied, but distinct, species, the true M. álba of Linnæus; which, although abundant in France, particularly in the neighbourhood of Calais, has never yet been discovered on the opposite shores of Kent, or in any part of England. As, therefore, our bird, which has always been considered as identical with the M. álba, proves to be a distinct species, I have named it after my friend W. Yarrell, Esq., as a just tribute to his varied talents as a naturalist.

The characters by which these two species may be readily distinguished are as follows:- The pied wagtail of England (M. Yarréllii) is somewhat more robust in form, and, in its full summer dress, has the whole of the head, chest, and back, of a full deep jet black; while in the M. álba, at the same period, the throat and head alone are of this colour, the back and the rest of the upper surface being of a light ashgrey. In winter, the two species more nearly assimilate in their colouring; and this circumstance has, doubtless, been the cause of their hitherto being considered as identical: the black back of M. Yarréllii being grey at this season, although never so light as in M. álba. An additional evidence of their being distinct (but which has, doubtless, contributed to the confusion) is, that the female of M. Yarréllii never has the back black, as in the male; this part, even in summer, being dark grey; in which respect it closely resembles the other species.

A third species of pied wagtail (the M. lugubris of Pallas) inhabits the eastern portions of Europe, and is rarely found in any other part of the Continent. In size, this species far exceeds M. Yarréllii: it has the whole of the centre of the wing, several of the secondaries, and the basal portion of some of the primaries, white; besides which, it may at all times be distinguished from either of the former species by the lorum, or space between the bill and the eye, being black; a character not to be found in any state of plumage in either M. Yarréllii or M. álba.

The wagtails constitute a well-defined and somewhat isolated group, are strictly confined to the older known portions of the globe, and may be said almost exclusively to inhabit the continents of Europe, Asia, and Africa; only two or three species having been found in the islands of the Indian Archipelago, and no example of the form having yet been discovered either in America or Australia. They appear to be naturally divided into two distinct sections; one characterised by a pied plumage, which is subject to a seasonal change, and with a short hind claw; the other by a more gay plumage, the colours of which are generally olive and rich yellow. In this latter section, the form is slighter in all its proportions, and the hind claw is much produced. The members of the two sections also differ as much in their economy as they do in external appearance; and I am of opinion that Čuvier's generic title of Budytes for the yellow section might be adopted with considerable propriety.

London, August 16. 1837.

ART. III. Illustrations of the Geology of the South-East of Dorsetshire. By the Rev. W. B. CLARKE, A.M. F.G.S.

(Concluded from p. 421.)

We must now revert to fig. 35. and fig. 36. Those sketches may be better represented geologically in the following section from Tillywhim Quarry (to the westward of Durlstone Head) to Ballard Down, taken from Phillips's and Conybeare's Outlines of the Geology of England and Wales.

In this figure (fig. 43.), the vertical and curved beds of fig. 37. are represented as connected with the general heave

of the inferior formations from under the chalk. Mr. Webster assumes, either that the beds of chalk were formed at two distinct periods, curved strata being laid over horizontal ones, as in fig. 44., and afterwards disarranged; or, as represented in fig. 45., where the whole of the strata are supposed to have been originally curved, the curves corresponding to similar curves, of which the section fig. 43. gives the

portions that remain. According to this supposition, it is evident that the chalk and inferior beds must have originally formed a dome over the whole of Purbeck; which, taking the height of Ballard Down to be, as he assigns, 584 ft., could not have been less than 4672 ft. thick. Such an enormous denudation as the present surface must have

required, on this supposition, is totally at variance with legitimate assumption; and, therefore, we are called on to reject the original curvature of the chalk altogether. The former idea of curved chalk strata over the slope of horizontal strata, as assumed in fig. 44., seems to me equally untenable; because, though undoubtedly the chalk was deposited by degrees, and may have been subjected to the action of currents of water, passing over each successive deposition, there is no example on record, that I am aware of, where the deposition of chalk was interrupted by any long interval, or where it attained a sufficient elevation above the sea to be subjected to denuding causes capable of so rounding and excavating the horizontal beds, and then became depressed, after that long interval, beneath the same depositing sea, whose action was again continued. That different modifications may have occurred in the different portions of the chalk formation, is, no doubt, consistent with facts; for how otherwise are we to