THE GASTROPODA 115 



marked, so that its development must have taken place 

 gradually within this family. 



In the Mesozoic rocks there is found another family, 

 the Nerinaida, some members of which have great super- 

 ficial resemblance to Centhium in form and ornament, so 

 that they can only be distinguished by taking vertical 

 sections through the shell (Fig. 34, i). It is then seen 

 that the cavity of each whorl is more or less constricted by 

 internal spiral ridges (not confined to the columella as any 

 are that occur in Centhium}. In extreme cases these 

 ridges are so developed that only a narrow space of com- 

 plicated form is left for the snail's body ; for instance, 

 in Ptygmatis, a common genus in the English Lower 

 Oolites. 



Rimella belongs to the family Strombida, other genera 

 of which have the outer lip expanded and sometimes 

 branching in a finger-like manner. The family ranges 

 from the Jurassic period to the Recent, and though 

 abundantly represented in the British area up to and in 

 the Eocene period, it is now confined to the Indo-Pacific 

 province, the Mediterranean and West Indies. The 

 closely-allied family Apovrhaida, however, with similar 

 digitations to the outer lip (Fig. 34, j}, and having the 

 same range in time, is essentially Atlantic in its modern 

 distribution, and the species Aporrhais pes-pelicani is 

 common in the British seas. 



Trivia and the Cypraida have already been mentioned. 



FIG 34. GASTROPODS (continued}. 



( X 3. ) (After Buvignier.) e, Nerita aperta J. de C. Sowerby, Oligo- 

 cene. ( X \. ) (After Edwards.) /, Capulus ungaricus (Linne") , Pliocene. 

 (Xj.) (After S. V. Wood.) g> Vivipara lenta (Solander), Oligocene. 

 (Xf. ) (After Deshayes. ) h, Melanatria inquinata (Defrance), 

 Eocene. (Xf.) (After Deshayes. ) i, Nerincea moreana d'Orbigny, 

 Upper Jurassic. (Xj.) (After Buvignier.) j, Aporrhais parkinsoni 

 Mantell, Gault. (Xj.) (After Starkie Gardner.) k, Nassa reticosa 

 (J. Sowerby), Pliocene. (X |.) (After S. V. Wood.) In several cases 

 the ornamentation is omitted from the earlier whorls for the sake of 

 clearness of outline. 



