bend back parallel to its former direction. Hamites is sometimes restricted to forms which have bent thus twice or three times during their growth, while those only once bent are styled Hamulina. Surface of most species rather simply ribbed.

Cretaceous.

Crioceras.-Form like an Ammonite, but evolute. Surface variously marked; but generally with strong simple ribs. See Ancyloceras.

Jurassic to L. Cretaceous.

Ancyloceras.-Form commencing like Crioceras; then becoming straight; and finally curving back along its inner side like the terminal part of Hamites. Hence Ancyloceras may be a late stage in the growth of Crioceras.

Jurassic to Lower Cretaceous.

Scaphites (fig. 117).-Form commencing as an involute shell; then running straight for a short distance; and finally curving back along its inner side. Commonly much stouter and more rotund than Ancyloceras. Aptychus or synaptychus known. Upper Cretaceous.

Turrilites.-Form spiral, generally lefthanded (ie., opposite to the mode of

Fig. 117. Scaphites coiling of typical gastropods).

æqualis (Cenomanian).

The

whorls are sometimes not in contact. Surface commonly marked with nodose ribs. The suture-lines of course readily distinguish this form from turreted gastropods; the cross-fracture of imperfect specimens has generally taken place along a septum, the characteristic ammonoid folding of which can at once be seen.

Cretaceous.

C. PHRAGMOPHORA.

This group is included by common consent under the Dibranchiate division of Cephalopods. In our selection of commonly occurring fossil types, we leave aside the very interesting series of forms intervening between Belemnites and the Cainozoic Sepia, which is abundant in existing seas. We cannot refrain, however, from a brief mention of such genera, following on the description of the stratigraphically important Belemnites.

Belemnites (fig. 118).-The chambered shell in this genus is reduced to a conical body, the Phragmocone (or Phragmacone),

which is divided internally by simple concave septa.

The

interseptal chambers are connected by a siphuncle, which runs down one side of the phragmocone, this being consequently called the ventral side. The phragmocone is closely fitted into a hollow, styled the Alveolus, which occurs at the anterior end of a strong pencil-like calcareous body, the Guard or Rostrum. This solid guard forms the object so commonly found, and is thus popularly known as the "belemnite."

The guard is of various proportions, sometimes delicately tapering, sometimes broad and stout, sometimes thickening anteriorly for a certain distance and then decreasing in diameter, to expand again as the alveolus is neared. In cross-section, as when broken, it shows a fibrous radial structure, and the calcite of which it is formed is usually stained somewhat brown. The apex of the conical alveolus is directed slightly to the "ventral" side, and determines the point from which the calcite prisms radiate in the guard; hence the axis of the guard is eccentric, and the "dorsal" or "ventral" side of imperfect specimens can be determined by noting which part of the circumference is respectively farthest from or nearest to the point from which the prisms radiate.

seen.

Fig. 118.- Guard

of

Belemnites, cut open above to show remains of the phragmocone resting in the alveolus.

The guard has typically a smooth surface, on which vascular impressions, like those of ramifying rootlets, can occasionally be A furrow sometimes runs down the ventral side, or more rarely down the dorsal, often reaching to the point of the guard. At the point itself furrows sometimes arise, extending some distance up the sides; and a common feature is the presence of two long and almost parallel grooves running thus up the dorsal side.

The alveolus is often empty; and sometimes the phragmocone is found without the guard. In fine and carefully cleaned specimens, not only can the phragmocone be seen in place, but traces of a broad expansion of its dorsal side extend considerably above and beyond it. This thin anterior expansion is the Pro-ostracum, and covers the ink-bag, the solidified contents of which, forming a black pear-shaped body, have also been found in situ. Above

* The Phragmocone itself has also been styled the Alveolus.

373

this, again, impressions of the crown of arms about the head of the animal may be seen, the little hooked teeth with which they were set lying in rows along them. Hence the "belemnite" familiar to collectors formed only the posterior hard part of an animal allied to our modern unprotected cuttle-fish.

Lower Lias to Albian.

Belemnitella (fig. 119). This form is practically only a subgenus of Belemnites, characterised by a slit at the anterior end of

Fig. 119.-Guard of Belemnitella mucronata (Senonian). Showing the slit and traces of vascular markings.

Fig. 120.-Guard of
Actinocamax plenus
(Belemnitella plena).
Cenomanian.

the guard and parallel to its axis. This slit reaches in to the alveolus, and marks the ventral side. The phragmocone (of which several specimens, mostly preserved in silica, are known), has a low ridge running down the dorsal side, a corresponding shallow groove occurring in the alveolus. The phragmocone, though rare, was described by Count Münster as early as 1830. The guard shows distinct vascular markings on its ventral surface. (Compare Actino camax.)

In

Upper Cretaceous. Actinocamax. this genus forms have been placed in which

no alveolus occurs, and also some with a shallow alveolus, which is often four-sided rather than circular in cross-section (Actinocamax quadratus). The common character of these forms is the distinctly lamellar structure of the anterior end of the guard, so that it easily becomes broken away and injured. In apparently perfect specimens, however, as in Actinocamax plenus (fig. 120; often styled Belemnitella plena), the anterior end may be pyra

midal, not hollowed out by an alveolus; in this case the phragmocone must have been surrounded by only a horny continuation of the guard. In the species quoted, a slight groove occurs at the apex, which may correspond to the slit in Belemnitella.

Both in Belemnitella and Actinocamax the guard may be suddenly reduced in diameter near its point, thus terminating in a short spinose process, the "mucro." It has often been

suggested that Actinocamax is only an imperfectly preserved Belemnitella; but the uniform character of specimens at certain horizons is evidence that the phragmocone was largely above, and not included in, the true calcareous guard.*

Upper Cretaceous.

Note.-Phragmophora with greatly elongated guards occur in the Upper Trias. In Belemnoteuthis of the Oxfordian, on the other hand, the guard is a mere short sheath about the phragmocone.

Belosepia of the Eocene has a short stout bent guard, expanded anteriorly, and protecting a curved phragmocone, a wide depression on the concave side of which does duty for a siphuncle. The pro-ostracum is large.

In Spirulirostra (Miocene) the guard forms a stout short process below a curved phragmocone, which possesses a true siphuncle. In the modern Spirula the phragmocone alone remains, in the form of a delicate evolute spiral shell, with a siphuncle on the concave side. This shell, though exposed by a cleft of the mantle, is truly internal.

Sepia (Eocene to Recent) has the merest trace of a guard at the end of a phragmocone, the chambers of which are flattened, and which forms the well-known "cuttle-bone." It is important to note, however, that cephalopods with a mere thin horny pro-ostracum (the "pen") over the ink-bag, and no trace of chambered shell or guard, occur as contemporaries of even the earlier belemnites. Thus Geoteuthis of the British Lias has been placed in the same group, the Chondrophora, as Loligo, the Squid of the present day.

* As to Belemnitella and Actinocamax, see Dr. Cl. Schlüter, "Cephalopoden der oberen deutschen Kreide," Paleontographica, vol xxiv. (1876-7), p. 63, and plate lii. (17), &c.

CHAPTER XXVI.

FOSSIL GENERIC TYPES.

XII. Echinodermata.

THE Echinoderms present in their hard parts a great variety of forms, characterised, however, by the prevalence of petagonal symmetry. The fact that their shells or skeletons, external or internal, are built up of plates, causes their remains often to be found only in a fragmentary condition. The calcite of which these parts is composed assumes a completely crystalline structure: so that any part of the shell or skeleton cleaves across on fracture along the rhombohedral surfaces so familiar in Iceland-spar. The opaque white but gleaming cleavage-surfaces of echinodermal fragments may thus be picked out by the eye on rock-exposures from among the fractured remains of other organisms. The individual plates or block-like calcareous bodies of which the hard parts are composed are styled the Ossicles. All the Echinodermata are Marine.

A. CRINOIDEA.

These are the typical "sea-lilies" or "encrinites." The animal during the whole or earlier part of its existence is fixed to the sea-bottom, commonly by a flexible stalk or Stem, which bears root-like processes at its base. The principal terms used in describing the hard parts of Crinoids are as follows :—

Ossicles or plates. The individual calcareous bodies of which any of the hard structures are built up.

Calyx.-The cup-like structure, sometimes closed over above, formed of calcareous plates, and enclosing the body of the animal. Its under surface or base, which is attached to the apex of the stem (or, as in Holopus, directly to the sea-floor), corresponds to the upper surface of most echinoderms; its upper or oral surface bears the mouth and generally the anal aperture. The calyx and the arms are often spoken of together as the Crown of the crinoid, and the calyx is sometimes freely termed the "head."

The upper (ventral) covering, or tegmen, of the calyx may be membranous, with little plates developed in it, thus leaving a circular gap in fossil forms; or it may form a dome-like structure of numerous plates in contact. There is no doubt that in some genera this dome was represented by a flexible ventral sac.