NATURAL SCIENCES OF PHILADELPHIA. 193 



Genus ABIOLIMAX, Morch. 



Jaw thick, slightly arcnate, ends but little attenuated, blunt ; 

 low, wide: anterior surface with numerous stout ribs, denticulating 

 either margin. The number of ribs varies 

 Fig 39. in the several species, and in different indi- 



viduals of the same species. Fig. 39, drawn 

 from the true northern A. Golumbianus^ has 

 18 ribs; another specimen, supposed to be 

 coiumuanns. ^^^ ^ame spccics, has about 12. (See Proc. 



Ac. Nat. Sc. Phila. 1874, pi. II., fig. 11.) A. 

 Calif ornicus has given 13 and 14 ribs. A. niger has been de- 

 scribed by Dr. Cooper with 20, but I found only 8 in one specimen 

 which I refer to that species. 



Fig. 498 of p. 279, L. and Frw. Shells N. A., I, gives the general 

 arrangement of the teeth upon the lingual membrane. It is drawn 

 from the true northern A. Columbianus. On pi. Y., fig. 6, I have 

 given more detailed figures of the dentition of a specimen I refer 

 to this species. It will be seen that the central teeth have a base 

 of attachment longer than wide, with expanded lower angles, and 

 incurved lower margin ; the upper margin is reflected ; the reflec- 

 tion is large, broad, and has a short, stout median cusp, bearing 

 a long, stout cutting point ; the side cusps of the reflection are 

 subobsolete, but there are well-developed triangular cutting points. 

 The laterals are like the centrals, but unsymmetrical by the sup- 

 pression of the inner lower lateral expansion to the base of attach- 

 ment, and the inner side cutting point, the inner side cusps being 

 still subobsolete. 



The change from lateral to marginal teeth is shown in b and c, 

 the median cusps and cutting point being greatly developed, and 

 sometimes (c) having a side cusp and cutting point ; the base of 

 attachment is still narrower than in the first laterals. The mar- 

 ginals are shown in d and e. They are about as high as wide, the 

 i-eflection equals the base of attachment and bears an extremely 

 long, blunt, stout, oblique cutting point, with a side spur upon 

 the last, in the extreme marginals developed into a short, stout, 

 side cutting point. The cutting point of the marginals by its 

 great development forms the chief characteristic of the membrane ; 

 it is well shown in profile (fig./.). There were 22 perfect laterals 

 in this specimen. The figui-e referred to above shows only 12 late- 

 rals, with 113 rows of 56 — 1 — 56 teeth each. 



