244 



HISTORICAL PALEONTOLOGY. 



Amongst the Jurassic Univalves (Gasteropoda} there are 

 many examples of the ancient and long-lived Pleurotomaria; 

 but on the whole the Univalves begin to have a modern 

 aspect. The round-mouthed (" holostomatous "), vegetable- 

 eating Sea-snails, such as the Limpets (Patellidfx}, the Nerites 

 (Nerita), the Turritellcc, Chemnitzia, &c., still hold a predomi- 

 nant place. The two most noticeable genera of this group 

 are Cerithium 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 Nerinaa as to 

 have gained the name of " Calcaire 

 a Nerinees. " 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 rig. 169. Nerincea Goodhaim, 



, (C , , , , one-fourth of the natural size. The 



SlpnonostomatOUS, applied to left-hand figure shows the appear- 



the shell. Some of the carnivorous 



forms belong to extinct types, such land. 



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 (Pteropoda},\t is sufficient to mark 

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



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. pompilius} ; 

 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, 



