248 



FORMS OF THE SPIRAL 



CHAP. 



Anostoma (Fig. 154), OpistJwstoma (Fig. 208, p. 309), Stroplio- 

 stoma^ and Hyj^selo stoma (Fig. 202 A, p. 302). 



Some genera of the Capulidae^ in which the shell is of a 

 broadly conical form or with scarcely any spire, develop an 

 internal plate or process which serves the purpose of keeping the 



Fig. 153. — Siliquaria anguina Lam. , show- 

 ing scalariform coil of upper wliorls and 

 irregular extension of the lower. 



Fig. 154. — Anostoma glohulosum Lam., 

 Brazil. (After'p. Fischer.) 



animal within the shell, and does the work of a strong attachment 

 muscle. In Mitndaria this process takes the form of a raised 

 horseshoe ; in Crucibidum it is cup-shaped, with the edge free all 

 round ; in Grolerus^ Ergaea^ Crepipatella, and Trochita we get a 



Fig. 155. — Various forms of the internal plate in Capulidae : A, Calyptraea (Mitru- 

 laria) equestris Lam., E. Indies; B, Cruclhulum scutellatum Gray, Panama; C, 

 Ergaea plana Ad., and Reeve, Japan; D, Galerus chinensis L., Britain; E, Crepi- 

 patella dilatata Lam., Callao ; F, Trochita maculata Quoy, N. Zealand ; G, Crepi- 

 dulafornicata Lam., N. America. 



series of changes, in which the edge of the cup adheres to the 

 interior of the shell, and then gradually flattens into a plate. In 

 Crepidida proper this plate becomes a regular partition, covering 

 a considerable portion of the interior (Fig. 155 G). JSipponyx 

 secretes a thin calcareous plate on the ventral surface of the foot, 



