THE NAUTILUS. 27 



depressed, quite regularly oval in shape, the left side being rather 

 more arcuate than the right; apex subacute, though not much ele- 

 vated behind the middle of the shell and decidedly turned to the 

 right ; the anterior slope is nearly rectilinear, the right and left 

 somewhat convex above, concave below and flattening out toward 

 the periphery ; the posterior slightly concave ; surface with the lines 

 of growth faint but quite regular, slightly rippled transversely or with 

 fine radial ribs (in two of the five specimens examined, radial ribs 

 are developed as strong as in A. fuse us eugraptus}. Length (fig. 1C) 

 6.5, width 4.5, alt. 1.5 mm. 



It will be observed that while this shell is larger and more de- 

 pressed than the typical obscurus, the proportion of the length to the 

 breadth is almost exactly the same. The longitudinal outline, how- 

 ever, is much nearer to Clessin's figure than it is Haldeman's. 



The affinities of this form are with A. fiiscus, kirklandi and penin- 

 sula. It is, however, more closely related to kirklandi than to the 

 others and possibly may prove to be a southern development of that 

 species. It differs, however, by its more depressed, narrower and 

 more regularly oval shell, and the peculiar concavo-convex outline of 

 the lateral slopes. From fuscus, it differs decidedly in contour by 

 reason of the more posterior, more prominent and more excentric 

 apex and the peculiar lateral slopes. The shape and color are so 

 entirely different, that there is no reason to confuse the i-ibbed form 

 with A. peninsulas, which is found associated with it. 



IV. ANCYLUS EXCENTRICUS Morelet (1851). PL 1, fig. 19-21. 



This species is the sole representative in the United States of a 

 group of general distribution in the West Indies, Mexico and Central 

 America, characterized by the prominent, rather obtuse and very 

 excentric apex and, usually, well-developed radial ribs over the sur- 

 face. The only recorded localities are Comal Creek, New Braun- 

 fels, and Barton Creek, Travis county, Texas. Specimens from the 

 latter locality, collected by Singley, are before me and are larger 

 than those from Comal Creek, cited by Pilsbry (NAUT. iii, p. 64), 

 and agree almost exactly with the dimensions given by Morelet, the 

 size of the specimen figured being, length 7^, width 4^, alt. 2 mm. 

 A. excentricus is so entirely different in shape from all other North 

 American species that there is no possibility of confusing it with any 

 of them. Another peculiarity of this species is the depression of the 



