shells, called Lituites. (Pl. 44, Fig. 3.) These are partially coiled up into a spiral form at their smaller extremity, whilst their larger end is continued into a straight tube, of considerable length, separated by transverse plates, concave outwards, and perforated by a siphuncle (a.) As these Lituites closely resemble the shell of the recent Spirula (Pl. 44, Fig. 2,) their office may have been the same, in the economy of some extinct Cephalopod.
Baculite.
As in rocks of the Transition series, the form of a straight Nautilus is presented by the genus Orthoceratite, so we find in the Cretaceous formation alone, the remains of a genus which may be considered as a straight Ammonite. (See Pl. 44, Fig. 5.)
The baculite (so called from its resemblance to a straight staff) is a conical, elongated, and symmetrical shell, depressed laterally, and divided into numerous chambers by transverse plates, like those in the Ammonite, are sinuous, and terrninated by foliated denotations at their junction with the external shell; being thus separated into dorsal, ventral, and lateral lobes and saddles, analogous to those of Ammonites.[1]
It is curious, that this straight modification of the form of Ammonites should not have appeared, until this Family had arrived at the last stage of the Secondary deposites, throughout which it had occupied so large an extent; and that, after a comparatively short duration, the Baculite
- ↑ The external chamber (a) is larger than the rest, and swelling; and capable of containing a considerable portion of the animal. The outer shell was thin, and strengthened, like the Ammonite, by oblique ribs. Near the posterior margin of the shell, the transverse plates are pierced by a Siphuncle (Pl. 44, 5b, c,). This position of the Siphuncle, and the sinuous form and denticulated edges of the transverse plates, are characters which the Baculite possesses in common with the Ammonite.
