NATURAL SCIENCES OF PHILADELPHIA. 181 



be seen below. It will be noticed that one species, asteriscus, has 

 marginal teeth like those of P^pa and Vertigo. 



Genus HEMITKOCHUS, Sw. 



In Ann. L3^c. N. H. of N. Y., X., 341, 1 have, in connection with 

 my friend Mr. Bland, shown the necessity of using this name in 

 preference to Polymita. 



The jaw is arched with acuminated ends, smooth anterior surface, 

 and decided median prominence to cutting margin. 

 Fig. 23. Fig. 23 represents the jaw of H. varians. The lin- 



gual membrane (pi. XIII., fig. 1) has about 33 — 1 — 33 

 teeth, another specimen gave 43 — 1 — 43 teeth, with 

 17 perfect laterals. The central tooth has a long, 

 narrow base of attachment with lower, outer angular 

 expansions and incurved lower margin. The reflected 

 portion is only about one-half the length of the base of attachment, 

 is short, and bears one short, stout cusp with an equally short, 

 stout cutting point ; the side cusps and cutting points are 

 obsolete. The laterals are the same as the centrals, but unsym- 

 metrical. The outer laterals have a side cusp and cutting point. 

 The marginals are low, wide, and have one broad, long, oblique, 

 bluntly bifid cutting point, the inner division the smaller, and 

 a very much shorter side cutting point. This side cutting point 

 is also sometimes bluntly bifid in the extreme marginal teeth. 



The dentition of the other species of this genus, extralimital to 

 North America, examined bj^ me, agrees with that of this species. 

 (See Pr. Phila. Ac. N. S., 1874, 56.) 



Genus TEBENNOPHORTJS, Binn. 



One species only is known to exist in North America, T. Caro- 

 liniensis. It has an arched jaw (fig. 24), with blunt, scarcely 

 attenuated ends, ribless anterior surface, and decided 

 Fig. 24. blunt median projection to the cutting edge. The 

 jaw is thick, coarse, with vertical and parallel trans- 

 verse lines of reinforcement, but has no appearance 

 of ribs. I have verified this fact by examining 

 Jaw of numerous specimens of all ages from various parts 



'^faZun^eZu,' of the country. My observations have been con- 

 firmed by Morse, also (Journ. Portland Soc. N. 

 H. 1864, 7). I am therefore inclined to doubt the identity of the 



