NATURAL SCIENCES OF PHILADELPHIA. 197 



great to raise a doubt of the specific identity of the two specimens. 

 The central teeth have a base of attachment longer than wide, 

 with short lines of reinforcement running parallel to the outer 

 edges at the lower margin. The upper margin is reflected. The 

 reflection extends about one-third of the length of the base of 

 attachment ; it bears a central, stout, well-developed cusp, and 

 one small, little-developed, rounded cusp at each side; all three 

 cusps have stout cutting points. The lateral teeth are like the 

 centrals, but unsymmetrical by the suppression of the inner cusp 

 and cutting point, and inner, lower, lateral expansion of the base 

 of attachment. The marginal teeth are low, wide; broadly re- 

 flected, the reflection equalling the length of the base of attach- 

 ment, and very irregularly denticulated, there being usually one 

 long, blunt, oblique, inner, bifid cutting point, the outer division 

 much the shorter, and several short, blunt, outer cutting points. 



P. Wetherbyi (pi. VI., fig. B), the only other species now known, 

 has 35 1 35 teeth, with 13 perfect laterals. The teeth are dif- 

 ferent from those of P. dorsalis, and nearer those of Tebennopho- 

 rus Caroliniensis. The side cusps of the centrals and laterals 

 are subobsolete, and have no distinct cutting points, the median 

 cusp is much more produced, stouter, and bears a stout, blunt, 

 cutting point. The marginal teeth are not so wide, they are less 

 irregularly denticulated, having usually one long, stout, blunt, 

 oblique, inner cutting point, and one shorter side cutting point. 



Genus HELIX, Lin. 



In common with all who have studied the genus Helix, I am 

 convinced of the necessity of recognizing among its species nu- 

 merous distinct genera. I have, however, at this time eliminated 

 those species only whose jaw has no distinct ribs upon its anterior 

 surface. 1 The balance of the species I retain grouped as sub- 

 genera 2 only. Before recognizing these groups as distinct genera, 

 I desire to wait until we can ascertain whether generic characters 



1 I fear that even this rule will not hold good. In some subgenera of 

 Helix the absence or presence of ribs on the jaw is not a reliable character. 

 For instance, in Dentellaria we have found the jaw of H. Josephinm and 

 formosa without ribs. H. dentiens, badia, pachygastra are heavily ribbed. 

 H. lychnuchus, nucleoid, and perplexa less so. 



2 In the list of species given above (p. 152), I have used only the sub- 

 generic names. 



