(J8 PKOSOBRANOHIATA. 



The shells of the Gasteropods, those elegant calcareous 

 cones moulded on the body of the animal, and justly prized 

 for their varied markings, exquisite sculpture, and graceful 

 contour of form, most frequently assume the shape of a spiral 

 coil. The spire is sometimes on the same plane, as in the 

 discoidal Coretus; or produced, as in the turreted Mega- 

 spira; or loosely coiled, as in the irregular Vermetus; or 

 unrolled and partly straight, as in the singular Campulotus ; 

 or it may be involute, as in the polished Cowry ; or ta- 

 pering at each end, as in the spindle-shaped Fusus ; or 

 with the last turn very large, as in the ear-shaped Haliotis; 

 or the coil may have a flattened base, as in the top- 

 shaped Trochus. When the shelly cone is not spirally 

 coiled, it is often tubular, as in the tooth-shaped Denta- 

 limn ; or flattened out, as in the shield-shaped Scufus ; 

 simply conical, as in the Limpet ; or slipper-shaped, as in 

 Cr^pta and Catlllus. The coils are generally right-handed, 

 but sometimes they are sinistral, as in Clausilia. The 

 marine spiral Gasteropods, which breathe by gills, usually 

 have the mouth of the shell closed when at rest by a 

 horny, or calcareous operculum ; the air-breathing families 

 are either provided with a similar means of defence, or are 

 entirely destitute of this appendage. 



Sub-class PROSOBRANCHIATA. 



Gills pectinate or plumose, placed in a mantle-cavity 

 above the neck, or under the mantle on the left side. 

 Heart situated behind the gills. Sexes distinct. Abdo- 

 men well developed, usually spiral, and protected by a shell. 

 Adult and larva shell-bearing ; larva furnished with de- 

 ciduous ciHated fins springing from the sides of the head. 



The Prosobranchiate Gasteropods comprise the Cuvierian 



