THE JURASSIC PERIOD. 



237 



eatiog Sea-snails, such as the Limpets (fatdlitfce), the Nerites 

 (Nerita), the Turritella, Cheinnitzite, &c., still hold a predomi- 

 nant place. The two most noticeable genera of this group 

 are Cerithiiim and Nerinaa the former of these attaining 

 great importance in the Tertiary and Recent seas, whilst the 

 latter (fig. 169) is highly characteristic of the Jurassic series, 

 though not exclusively confined to 

 it. One of the limestones of the 

 Jura, believed to be of the age of 

 the Coral-rag (Middle Oolite) of Bri- 

 tain, abounds to such an extent in 

 the turreted shells of Nerincea as to 

 have gained the name of " Calcaire 

 a Nerine'es." In addition to forms 

 such as the preceding, we now for 

 the first time meet, in any force, 

 with the Carnivorous Univalves, in 

 which the mouth of the shell is 

 notched or produced into a canal, 

 giving rise to the technical name 

 of " siphonostomatous," applied to 

 the shell. Some of the carnivorous 

 forms belong to extinct types, such 

 as the Purpuroidea of the Great Oo- 

 lite ; but others are referable to well-known existing genera. 

 Thus we meet here with species of the familiar groups of the 

 Whelks (Buccinum), the Spindle - shells (Fusus), the Spider- 

 shells (Pteroceras], Murex, Rostellaria, and others which are 

 not at present known to occur in any earlier formation. 



Amongst the Wing-shells (Fieropodd}, it is sufficient to mark 

 the final appearance in the Lias of the ancient genus Comdaria. 



Lastly, the order of the Cephalopoda, in both its Tetrabran- 

 chiate and Dibranchiate sections, undergoes a vast devel- 

 opment in the Jurassic period. The old and comparatively 

 simple genus Nautilus is still well represented, one species 

 being very similar to the living Pearly Nautilus (N. pompihus); 

 but the Orthocerata and Goniatites of the Trias have finally 

 disappeared; and the great majority of the Tetrabranchiate 

 forms are referable to the comprehensive genus Ammonites, 

 with its many sub-genera and its hundreds of recorded species. 

 The shell in Ammonites is in the form of a flat spiral, all the 

 coils of which are in contact (figs. 170 and 171). The inner- 

 most whorls of the shell are more or less concealed ; and the 

 body-chamber is elongated and narrow, rather than expanded 

 towards the mouth. The tube or siphuncle which runs through 

 '17 



Fig. 169. AVr/<ZYZ Goodhallii, 

 one-fourth of the natural size. The 

 left-hand figure shows the appear- 

 ance presented by the shell when 

 vertically divided. Coral -rag, 

 England. 



