OF MOLLUSCA. 9 



procal impregnation. Shell spiral. Adult and larva shell-bear- 

 ing ; larva with deciduous cephalic fins. 



Order IV. Gymnobranchiata. Gills exposed or contractile into a 

 cavity on the surface of the mantle. Adult shell-less ; larva shell- 

 bearing, with deciduous cephalic fins. Animal hermaphrodite, 

 with reciprocal impregnation. 



Order V. Pneumobranchiata. Respiring free air in a closed 

 chamber lined with pulmonic vessels. Adult and larva shell- 

 bearing ; larva shaped like the parent, without cephalic fins. 



Subclass I. CTENOBRANCHIATA, Gray. 



Gills comb-like, formed of one or two series of plates (or some 

 ramified vessels) on the inner surface of the mantle (or rarely round 

 its edge). Gill-cavity open. Mantle fr^ee from the back of the neck. 



Adult and larva shell-bearing ; larva furnished with deciduous 

 ciliated fins on the side of the head. Unisexual or hermaphrodite 

 and self-impregnating. 



Order I . Pectinibranchiata, Cuvier. 



Gills comb-like, formed of one or rarely two longitudinal series 

 of lamellae (or very rarely some branched vessels) on the left side of 

 the mantle over the back of the neck. Gill-cavity open, the mantle 

 edge being free from the back of the neck. Animal unisexual. Shell 

 spiral. Operculum usually distinct. 



Synopsis of Suborders. 



I. Toxifera. Head small ; proboscis retractile under the base of 



the tentacles. Teeth elongate, subulate, implanted in the fleshy 

 proboscis (figs. 2,3). 



II. Proboscidifera. Head small ; proboscis retractile under the 



base of the tentacles. Teeth variable, in an elongated cartilagi- 

 nous tongue-membrane (figs. 1, 6, 7, &c). 



III. Rostrifera. Head produced into a rostrum, with the ten- 

 tacles on the side of its base. Teeth in seven longitudinal series 

 3 • 1 * 3, in a cartilaginous tongue-membrane (fig. 1 ) . 



Fig. 1. — Lingual membrane and teeth of Buccinwn undatum. 



a, the front end of lingual hand. r, central teeth. /. lateral teeth. 



B 2 



