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226 PROCEEDINGS OF THE ACADEMY OF 
not able to give a figure of the jaw of the only species found 
within our limits, LZ. fasciatus. It is however figured by Leidy 
(Terr. Moll. U. S., L., pl. V., fig. 4, a,b). It is similar to that of 
the allied species LZ. virgineus, which is here figured on p. 225. 
The only species found within our limits, Z. fasczatus, has about — 
69—1—69 teeth, judging from a membrane examined by me. _ 
That figured in L. and Frw. Shells, L., 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 (pl. VL, fig. KE, 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 very 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 shouid, 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 pl. VLI., fig. EH, 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, UZ. vergineus, 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. 
