NATURAL SCIENCES OF PHILADELPHIA. 205 



Centrals with a base of attachment longer than wide, the lower 

 lateral angles but little expanded, the lower margin incurved, the 

 upper 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 unsymmetrical by the suppression of the inner lateral angle 

 of the lower edge of the base of attachment and the inner side 

 cusp and cutting point. The transition from laterals to marginals 

 is shown in pi. IX., fig. 8 {H. spinosa). It is, as usual, produced 

 by the comparative lesser development of the inner eusp 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 equalling 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 clenticulation of the mar- 

 ginal teeth even on the same lingual membrane. A transition from 

 laterals to marginals similar to that of H. spinosa is found in H. 

 barbigera, Edvardsi, stenotrema, hirsuta, germana, and monodon. 



There seems no difference in the characters of the teeth of the 

 different species examined by me, excepting the slight one of the 

 greater or lesser development of the side cusps of centrals and 

 laterals, especially the former ; whether this is constant can only 

 be proved by a careful examination of every portion of each lin- 

 gual. In H. hirsuta I found these cusps more developed than in 

 the other species (pi. IX., fig. 6). 



The count of the teeth in the different species is as follows : 



H. spinosa (pi. IX., fig. 8) has 27 1 2Y teeth; 9 perfect 

 laterals. 



H. Edvardsi (pi. IX., fig. 1) has 20120 teeth; 9 perfect 

 laterals. 



H. barbigera (pi. IX., fig. 9) has 21121 teeth; 8 perfect 

 laterals ; but even the third has its inner cutting point greatly 

 produced. 



H. stenotrema (pi. IX., fig. 1) has 20 120 teeth ; 10 perfect 

 laterals. 



