196 STRUCTURE OF THE MOLLUSCA. 



In the Cyprceina, the aperture is extremely narrow, arul 

 bordered with plaits or teeth on both sides. The Ooulce 

 are similar, but with teeth only on one side of the aperture. 



The Cones have the shell of an obconical form, and the 

 aperture very narrow, without plaits or teeth. 



Many of the Pectinibranchiata have shells in the form of 

 a shield, cup, or hollow cone, and have been called Cupaloid. 

 Of this kind are the genera Plleopsis, of a conical form, with 

 the apex a little spiral; Crepidula, of an oval form, with the 

 aperture half closed by a horizontal plate, which supports 

 the abdominal sac, and is covered beneath by the foot ; 

 CalyptrcBa, having in the hollow of their conical shell, a 

 small projecting lamina, looking like the commencement of 

 a columella; Sigaretus, in which the shell is concealed by 

 the mantle, the aperture very large, and the spire small. 



OEDEK III.-GASTEEOPODA TUBULIBEAXCHRTA. 



The Tubulibranchiate Gasteropoda have a great resem- 

 blance to the Pectinibranchiate species, but are distinguished 

 by very important differences in their organization, as well 

 as by their shell, which is fixed to submarine bodies, and 

 has the form of a more or less irregular tube, of which the 

 commencement only is spirally bent. Vennetus, Mayilus, 

 and Siliquaria, belong to this order. 



OEDEE IV -GASTEEOPODA SCUirBEAXCHIATA. 



The animals of this order have the shell very widely open, 

 more or less scutiform, and covering the branchiae, or even 

 the whole body. In Haliotis, the shell is slightly convoluted, 

 with the spire very small and flattened, the aperture exces- 

 sively large. Along the columella is a series of holes by 



