24*2 MOLLUSCA. 



PHYSA, Drap. 



The Physa have a shell very similar to that of a Lymnaeus, but without the 

 fold in the columella and reflected edge, and very thin. When the animal 

 swims or crawls, it covers its shell with the two notched lobes of its mantle, 

 and has two long, slender, and pointed tentacula, on the greatly enlarged 

 internal base of which are the eyes. They inhabit springs, &c. 



AURICULA, Lamarck. 



Differing from all the preceding aquatic Pulmonea in the columella, which is 

 marked with wide and oblique flutes. Their shell is oval or oblong, the aper- 

 ture elevated as in Bulinus, and the margin tumid. 



CONOVULUS, Lamarck. 



Projecting folds in the columella, as in the Auricula?, but the margin of the 

 aperture is not tumid, and the internal lip is finely striated ; the general form 

 of the shell is that of a cone, of which the spire forms the base. They 

 inhabit the rivers of the Antilles. 



ORDER II. 

 NUDIBRANCHIATA. 



THE Nudibranchiata have no shell whatever ; neither are they furnished 

 with a pulmonary cavity, their branchiae being exposed on some part of the 

 back. They are all marine animals, frequently swimming in a reversed 

 position with the foot on the surface, concave like a batteau, and employing 

 the margin of their mantle and their tentacula as oars. In the 



DORIS, Cuvier, 



The branchiae are arranged in a circle round an opening in the posterior part 

 of the back, under the form of little arbusculte, the whole resembling a sort 

 of flower. The mouth is a small proboscis, situated under the anterior margin 

 of the mantle, and furnished with two little conical tentacula. The species 

 are numerous, and some of them large. They are found in every sea, 

 where their ova, resembling gelatinous bands, are diffused over stones, sea- 

 weeds, &c. 



PLOCAMOCEROS, Lcuck. 



The anterior margin of their mantle is ornamented with numerous branched 

 tentacula. 



POLYCERA, Cuvier. 



The branchiae, as in Doris, on the hind part of the body, but more simple, 

 and followed by two membranous lamella-, which cover them in moments of 



