Fig. 89.—Pentacrinites Briareus. Half natural size.
Among the Echinodermata belonging to the Lias we may cite Asterias lumbricalis and Palæocoma Furstembergii, which constitutes a genus not dissimilar to the star-fishes, of which its radiated form reminds us. The Pentacrinites, of which Pentacrinites Briareus is a type, ornaments many collections by its elegant form, and is represented in [Figs. 79] and [89]. It belongs to the order of Crinoidea, which is represented at the present time by a single living species, Pentacrinus caput-Medusæ, one of the rare and delicate Zoophytes of the Caribbean sea.
Oysters (Ostrea) made their appearance in the Muschelkalk of the last period, but only in a small number of species; they increased greatly in importance in the Liassic seas.
The Ammonites, a curious genus of Cephalopoda, which made their first appearance in small numbers towards the close of the preceding Triassic period, become quite special in the Secondary epoch, with the close of which they disappear altogether. They were very abundant in the Jurassic period, and, as we have already said, each zone is characterised by its peculiar species. The name is taken from the resemblance of the shell to the ram’s-horn ornaments which decorated the front of the temple of Jupiter Ammon and the bas-reliefs and statues of that pagan deity. They were Cephalopodous Mollusca with circular shells, rolled upon themselves symmetrically in the same plane, and divided into a series of chambers. The animal only occupied the outer chamber of the shell; all the others were empty. A siphon or tube issuing from the first chamber traversed all the others in succession, as is seen in all the Ammonites and Nautili. This tube enabled the animal to rise to the surface, or to sink to the bottom, for the Ammonite could fill the chambers with water at pleasure, or empty them, thus rendering itself lighter or heavier as occasion required. The Nautilus of our seas is provided with the same curious organisation, and reminds us forcibly of the Ammonites of geological times.
Shells are the only traces which remain of the Ammonites. We have no exact knowledge of the animal which occupied and built them. The attempt at restoration, as exhibited in [Fig. 91], will probably convey a fair idea of the Ammonite when living. We assume that it resembled the Nautilus of modern times. What a curious aspect these early seas must have presented, covered by myriads of these Molluscs of all sizes, swimming about in eager pursuit of their prey!
Fig. 90.—Ammonites Turneri, from the Lower Lias.
The Ammonites of the Jurassic age present themselves in a great variety of forms and sizes; some of them of great beauty. Ammonites bifrons, A. Noditianus, A. bisulcatus, A. Turneri ([Fig. 90]), and A. margaritatus, are forms characteristic of the Lias.