LAMARCK'S 



GENERA OF SHELLS. 



ARRANGED ACCORDING TO THE DESCENDING SCALE, WITH THE 

 ADDITION OF SOME RECENTLY FORMED GENERA. 



TESTACEOUS MOLLUSCA. 



Animals soft, inarticulated, provided with an anterior head, 

 which is more or less projecting, or salient ; most frequently with 

 eyes, and furnished with tentacula, or possessing at their sum- 

 mit, arms disposed in the form of a coronet ; their mouth either 

 short, elongated, or tubular, extensile and usually armed with 

 hard parts. Mantle diversified, having its edges free on the sides 

 of the body, or the lobes united, forming a sack, which partly en- 

 velopes the animal ; gills or organs of respiration various, rarely 

 symmetrical; circulation double, one particular, the other general ; 

 heart unilocular, sometimes with the auricles divided, and very 

 distant, no medullary cord along the body, but provided with a 

 few scattered nerves and ganglions : body sometimes naked, 

 either unprovided with solid internal parts, or inclosing a shell 

 or other hard substance ; provided vv'ith an external shell cover- 

 ing, or sheathed in the body, and which is never composed of 

 two opposite valves. 



ORDER L— HETEROPODA. 



Head distinct, with two eyes, but destitute of arms arranged 

 around the head ; body free, elongated, fitted for swimming hori- 



