272 A MANUAL OF AMERICAN LAND SHELLS. 



Jaw tbick, high, arched; ends but little acnininated, blunt; cutting 



i^^^^s- margin without median projection ; anterior surface 



with stout, broad, crowded ribs, denticulating either 



^ margin. There are about 8 in stenotremum, 11 in ger- 



'^''^ fMOTseT"'*''"' manum, 7 in monodon, 8 in hirsutum, 13 in Udmrdsi, 12 



in barhigerunij 8 in spinosum, 12 in lahrosum. 



I have had no opportunity of examining Edgarianum ot maxillatum. 



The subgenus is restricted to North America, as far as known. It 

 differs from our other subgenera in having the ribs on its jaw much 

 broader and more closely crowded. 



Lingual membrane arranged as in Pafula. Centrals with a base of 

 attachment longer than wide, the lower lateral angles but little ex- 

 panded, the lower margin incurved, the uiiper margin squarely reflected; 

 reflection large, wide, with small, in some species almost obsolete, side 

 cusps, always bearing distinct, well developed cutting points, and a 

 very stout median cusp, bearing a stout cutting point, which usually 

 projects beyond the lower edge of the base of attachment. Laterals 

 like the centrals, but asymmetrical by the suppression of the inner 

 lateral angle of the lower edge of the base of attachment and the inner 

 side cusi) and cutting point. The transition from laterals to marginals 

 is shown in Terr. Moll., V, Plate VII, Fig. B {8. spinosum). It is, as 

 usual, produced by the comparative lesser development of the inner 

 cusp and greater development of its cutting point. This cutting point 

 becomes bifid, the reflection becomes shorter, the cutting points more 

 produced, and thus gradually the form of the marginal teeth is reached. 

 They are low, wide, the reflection equaling the base of attachment, 

 the cutting points long, oblique, usually two in number, the inner one 

 generally and the outer one rarely bluntly bifid; the outer bifurcation 

 of each is more produced than the inner. There is great variation in 

 the denticulation of the marginal teeth even on the same lingual mem- 

 brane. A transition from laterals to marginals similar to that of S. 

 npinosum is found in 8. harhigerum^ lahrosinn, JEdvardsi, stenotremum, 

 hirsutumj germanum, and mcnodon. There seems no difference in the 

 characters of the teeth of the different species examined by me, except- 

 ing the slight one of the greater or lesser development of the side cusps 

 of centrals or laterals, especially the former ; whether this is constant 

 can only be proved by a careful examination of every portion of each 

 lingual. In S. hirsutum I found these cusps more developed than in 

 the other species. 



