226 PROCEEDINGS OP THE ACADEMY OP 



not able to give a figure of the jaw of the only species found 

 within our limits, L.fasciatus. It is however figured by Leidy 

 (Terr. Moll. U. S., I., pi. V., fig. 4, a, b). It is similar to that of 

 the allied species L. virgineus, which is here figured on p. 225. 



The only species found within our limits, L.fasciatus, has about 



69 l 69 teeth, judging from a membrane examined by me. 



That figured in L. and Frw. Shells, I., p. 214, has 94 rows of 55 



1 55 teeth each. As elsewhere stated, there is often a difference 



in the number of transverse teeth in almost all species, and indeed 

 upon different parts of the same membrane. 



The central tooth (pi. VI., fig. E, a) has a base of attachment 

 long and narrow, with strongly incurved sides, widely expanded, 

 excurved and fringed lower margin, and upper margin less ex" 

 panded, rounded, and broadly reflected. The reflection is stout 

 and very rapidly narrows without any appearance of side cusps 

 into a ver}' broad, long, bluntly rounded median cusp, bearing a 

 still broader, short, bluntly truncated cutting edge (as such a 

 blunt organ cannot be called a point) reaching nearly to the lower 

 edge of the base of attachment. It may be that I have here in- 

 correctly considered the upper margin of the base of attachment 

 as reflected and extended into the cusp. As in the case of the 

 side teeth, I should, perhaps, rather say that the upper margin is 

 not reflected, but that just below the middle of the base of attach- 

 ment there springs up from its surface a broad, gouge-shaped 

 cusp, bearing a still broader cutting edge (see pi. VI., fig. E, d, 

 where the form of the cusp of the side teeth is shown by the pro- 

 file). The side teeth run rapidly and obliquely backward from 

 the central tooth, thus giving a chevron-like arrangement to the 

 membrane. The teeth are crowded together both longitudinally 

 and transversely, excepting as they approach the outer edges of 

 the membrane, where they are much more separated. 



I have used the term side teeth instead of lateral and marginal 

 teeth, because it is difficult to decide which of these types they 

 properly are. Taking into consideration the fact of there being 

 distinct lateral teeth in the allied species, L. virgineus, and that 

 the marginals of that species resemble the side teeth of L.fas- 

 ciatus, I am inclined to believe we should consider all the side 

 teeth of fasciatus as marginals. In this case we must consider 

 that the lateral teeth are entirely suppressed. The marginals, as I 

 have decided to call them, are of the same type as the centrals. 



