MOLLUSCA. 177 
The species of this genus have not been sufficiently in- 
vestigated in a living state. When preserved in spirits, it 
is impossible to form a correct idea of their true appearance, 
as exhibited when alive in sea-water, since they usually exist 
as a shapeless mass. Cuvier has given delineations of such 
preserved species, but they bear no resemblance to the figures 
of Montagu, of the same species, taken from living objects. 
M. Lamark is inclined to divide the genus into two, distin- 
guishing those who have the shell concealed, by the term 
Bulla, from such as have the shell in part exposed, which 
he retains in the genus Bulla. The shells of the genus 
Bullea are thin and white, as B. aperta; those of Bulla 
stronger, more opake, and covered with an epidermis, which, 
after the death of the animal, is easily detached, as B. lig- 
naria. 
Genus Doripium (of Mekel). Destitute ofa dorsal plate 
or shell. There is a cavity in the cloak, with a spiral turn. 
The branchiz, and accompanying organs, are placed far be- 
hind. There is here no appearance of a spinous tongue ; 
the gullet is simple, and the stomach is membranaceous. D. 
carnosum, a native of the Mediterranean, is the type of the 
genus. 
OrperR II.—BRANCHIE INTERNAL. 
The aérating organs are contained in a cavity, and appear 
in the form of sessile, pectinated ridges. 
Ist Subdivision. 
Heart entire, and detached from the rectum. 
This group, forming the Pectinibranchia of Cuvier, in 
cludes nearly all the marine Gasteropoda, which have spiral 
