212 GASTEROPODA. 



Class GASTEROPODA. 



Head distinct, furnished with eyes and tentacles. Body usually 

 protected by a conical or spiral shell. Lower part of body formed 

 into a thickened, expanded, creeping disk or foot. 



Sub-Class OPISTHOBRANCHIATA. 



Gills exposed, or only slightly covered by a fold of the mantle 

 situated behind the heart, and never lodged in a distinct cervical 

 cavity. Sexes united in the same individual. Abdomen rudiment- 

 ary, not spirally developed in the adult, or protected by a shell. 

 Larva shell-bearing, and furnished with deciduous cephalic fins. 



Order TECTIBRANCHIATA.* 



Gills forming a tuft or plume on one side, towards the hind part 

 of the Ijody, under a fold of the mantle, and usually protected by a 

 shell. Both the adult animal and larva shell-bearing. Foot elon- 

 gate, formed for walking. Marine. 



Family BULLID^, D'Orbigny. 



Shell thin, usually rolled up like a scroll ; animal destitute of 

 tentacula, and having the branchia3 in a special cavity, at the pos- 

 terior part of the back, covered by the mantle. 



* In Dr. Gould's MS. the only work done on this order was pasting on separate 

 sheets the descriptions of each species of the first edition, and adding a sheet for each ad- 

 ditional species which he wished represented in the second edition. These additional 

 sheets I have tilkd up with a copy of the original description of each species. In case 

 of both new and old s])ecies, I have added the notes in Dr. Gould's interleaved copy of 

 the first edition. The only thing for which I am alone responsilile is the synonymy 

 and the generic descriptions, which last are copied from the " Genera of Recent 

 Mollusca." — W. G. B. 



