204 



PROCEEDINGS OF THE ACADEMY OF 



reflected : reflection large, wide, with distinct, but small, rounded 

 side cusps bearing short conical cutting points, and a very stout 

 median cusp reaching the lower margin of the base of attachment, 

 be3'ond which projects the short, stout, conical cutting point. 

 Laterals like the centrals, but unsymmetrical by the suppres- 

 sion of the inner, lower angle of the base of attachment, and the 

 inner side cusp and cutting point. First marginals a simple 

 modification of the laterals by the lesser development of the cut- 

 ting point (b). Outer marginals (c) low, wide, the reflection 

 equalling the base of attachment and bearing one inner, short, 

 stout, oblique cutting point, and two shorter outer blunt cutting 

 points. 



Polygyrella is quite distinct from all the other American sub- 

 genera of Helix by the form of its jaw and the large number of 

 ribs upon its anterior surface. 



Subgenus Stenotrema. 

 Jaw thick, high, arched; ends but little acuminated, blunt; 

 cutting margin without median projection ; anterior surface with 

 stout, broad, crowded ribs, denticulating either 

 margin. There are about 8 in stenotrema, II in 

 germana^ 7 in monodon, 8 in Jiirsuta, 13 in 

 Edvardsi, 12 in harhigera^ 8 in spinosa. 



I have had no opportunity of examining H. 

 labrosa, Udgariana, or maxillata. 



The subgenus is restricted to North America 

 It ditfers from our other subgenera in having 



Fiff. 47. 



Jaw of Helix monodon 

 [Morse ] 



as far as known. 



its ribs much broader and much more closely crowded. 



Fig. 48. 



Lingual dentition of Helix monodon. [Morse.] 



Fig. 48, drawn by Mr. Morse, gives the general arrangement of 

 the teeth on the lingual membrane. The characters of the indi- 

 vidual teeth are more correctly shown in my figures on pi. IX. 



1 See Ann. Lye. N. H. N. Y., X. pi. XIV., fig. 4. Perhaps a Mesodon. 



