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 

 Bullcea, from such as have the shell in part exposed, which 

 he retains in the genus Bulla. The shells of the genus 

 Bullcea 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 DORIDIUM (of Mekel). Destitute of a dorsal plate 

 or shell. There is a cavity in the cloak, with a spiral turn. 

 The branchiae, 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. 



ORDER II. BRANCHIAE INTERNAL. 



V 



The aerating organs are contained in a cavity, and appear 

 in the form of sessile, pectinated ridges. 



1st Subdivision. 



Heart entire, and detached from the rectum. 



This group, forming the Pectinibranehia of Cuvier, in 

 eludes nearly all the marine Gasteropoda, which have spiral 



