UMAECK'S GENEKA 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 furnished with eyes and tentacula, or sometimes 

 having arms disposed in the form of a coronet ; their mouth 

 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 sac 

 or bag, which partly envelopes the animal; gills or organs 

 of respiration various, rarely symmetrical ; circulation double, 

 one partial, the other general ; heart unilocular, sometimes 

 with the auricles separated, and very distant, no continuous 

 medullary cord along the body, but a few scattered nerves 

 and ganglions; body sometimes naked, either unprovided 

 with solid internal parts, or enclosing a shell or other hard 

 substance ; but generally provided with an external shell 

 covering the body, or sheathed in it, 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 

 f3 



