112 PALEONTOLOGY 



a flat base and an aperture wider than high (Fig. 34, d) ; 

 Turbo (Sil.-Rec.) differing from Trochus in having a convex 

 base and a nearly circular aperture, so that if placed apex 

 downwards it is seen to resemble a spinning-top : this 

 form is described as turbinate ; Nerita (Jur.-Rec.), globose, 

 imperforate with semicircular aperture and thick, often 

 grooved, lips ; Neritina (Eoc.-Rec.)j similar, but with thin 

 shell and lips, a fluviatile snail (Fig. 34, e). 



ORDER: CTENOBRANCHIA. 



Divided into a number of sub-orders on a basis of 

 tongue-structure, the only convenient palaeontological 

 division is into holostome and siphonostome forms. 



(a) Holostome. Scalaria (Trias.-Rec.) is turreted, 

 with circular aperture, strong, sharp vertical ridges 

 and finer spiral lines. Solarium (Jur.-Rec.) is conical, 

 with obtuse spiral angle, umbilicus very wide and deep, 

 and four-sided aperture. The nucleus (or larval shell) is 

 heterostrophic, as in PlanovUs, as can easily be detected on 

 a well-preserved specimen. 



Littorina (Jur.-Rec.), the periwinkle, somewhat re- 

 sembles Turbo in form, but is not nacreous. Natica and 

 Turritella have already been described. Vivipam, or 

 Paludina (Jur.-Rec.), is turbinate with rounded apex, 

 rounded whorls and a sub-circular aperture : it is a fresh- 

 water form (Fig. 34, g). 



The family Capulida is an interesting case of de- 

 generation : it has taken to microscopic food and a 

 fixed habit ; in consequence the shell reverts to a more 

 or less limpet-like form. There is therefore some un- 

 certainty as to whether certain fossils belong to this 

 family or to the Patellidce. Capulus (Fig. 34, /) shows 

 a trace of spiral curvature at the apex ; Crepidula 

 (Cret.-Rec.), the slipper-limpet, has a slipper-shaped 

 shell, and forms arch-like colonies, one individual on 



