402 TERRESTRIAL AIR-BREATHING MOLLUSKS. 



is, however, figured by Leidy (Vol. I. PI. V. Fig. 4, a, b). It is similar to that 

 of the allied species L. virgineus, which is figured here on last page. 



The only species found within our limits, L. fasciatus, has about 69 — 1 — 69 

 teeth, judging from a lingual membrane examined by me. That figured in L. 

 and Fr.-W. Sh., 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 mem- 

 brane is shaped like that of Orthalicus. (See PI. XVI. Fig. M.) 



The central tooth (PI. X. Fig. G) ha3 a base of attachment long and nar- 

 row, with strongly incurved sides, widely expanded, excurved and fringed 

 lower margin, and upper margin less expanded, 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 incorrectly 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 attachment there springs up from 

 its surface a broad, gouge-shaped cusp, bearing a still broader cutting edge (see 

 d, where the form of the cusp of the side teeth is shown by the profile). The 

 side teeth run rapidly and obliquely backward from the central tooth, thus 

 givincr 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. fasciatus, 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. The base of attachment is, however, asymmetrical 

 by the suppression of both upper and lower inner lateral expansion; the upper 

 margin is simply squarely truncated. Above the centre of the base of attach- 

 ment springs from its surface the gouge-shaped, rounded, gradually expanding 

 cusp, reaching nearly the lower margin of the base of attachment, and produced 

 into a still more expanded, bluntly truncated cutting edge (one cannot call it a 

 cutting point), which projects far beyond the lower margin of the base of 

 attachment on to the teeth of the next transverse row, and is also greatly ex- 

 panded on the outer side, so as to overlap the adjoining tooth. This cutting 

 edge is slightly incurved at its centre. There is one point of difference be- 



