ZOOLOGY. 783 



III. The Vitrinidae may be considered as embracing the Monotrema- 

 tous Geopbila, witb a jaw which is smooth or striated (destitute of ribs) 

 and has a median projection to the cutting edge; teeth diiaerentiated 

 into a central tricuspid one, " hiterals of about the same height as cen- 

 trals, bicuspid or tricuspid, but in latter case furnished with an ob- 

 solete inner cusp ; marginal teeth differing from the laterals, aculeate 

 unicusi)id or bicuspid," and a heliciform shell. This family embraces 

 about fifty-two species, which are referred by Mr. Binney to the Lima- 

 cidse. 



IV. The BulimulidaB include forms which have the jaw thin, and pro- 

 vided with distant transverse ribs, the teeth " peculiar by the elongation 

 and incurvation of the ini'er cusp of the lateral " ones, and the mantle 

 included in the shell which, itself is more or less elongated and turreted. 

 Nine species, referred to the genus Bulimulus, are represented in North 

 America, and are chiefly found in the southwestern regions. 



V. The StenogyridsB include forms which have the "jaw ribbed or 

 finely wrinkled, thin, arched;" the teeth differentiated into (1) a small 

 central tooth, (2) tricuspid lateral teeth having the " central cusp long 

 and narrow" and "side cusps of subequal length," and (3) "marginal 

 teeth, quadrate, very low, wide," and " tricuspid or multifid ; " the man- 

 tle included, and the " shell generally elongated, polygyral, shining," 

 and with its " apex more or less obtuse." The six North American 

 species are referred to the genera Stenogyra, Ferrussacia, and Caecilian- 

 ella. 



VI. The Helicidie have a jaw of a single piece (generally arcuate, 

 often ribbed), teeth diiferentiated into (1) "centrals unicuspid or tricus- 

 pid,"(2)" laterals unicuspid, bicuspid, or tricuspid, but with theinnercusp 

 obsolete," and (3) "marginals quadrate, low, wide;" the mantle sub- 

 median and included in the shell, and the shell spiral and diversiform. 

 The American representatives are distributed by Mr. Binney among 

 two families, the Helicidae and Pupidae, but the only differences noticed 

 are as to degrees of exsertion of the spire. The gradations from a dis- 

 coid helix to an elongated turreted or pupiform shell are so gradual 

 and numerous (especially when foreign forms are taken into account) 

 that no advantage seems to result from the distinction of the two groups. 

 Including both types and eliminating the genera Arion, Ariolimax, Pro- 

 physaon Binneya, Hemphillia, and the Punctum pygmaeum or minutis- 

 simum, one hundred and sixty-seven species have been admitted by Mr. 

 Binney into the North American fauna and distributed among twenty- 

 six genera. 



VII. The LimacidsB may be restricted to species having the jaws rib- 

 less, the dentition divided into (1) a " central tooth tricuspid," (2) lat- 

 erals of same height as central, bicuspid or tricuspid, and (3) " mar- 

 ginal teeth differing from the laterals, aculeate, unicuspid or bicuspid," 

 the mantle anterior, small, narrow, and shield-like in appearance, and 

 the shell reduced to a rudiment and concealed under the mantle. All 



