THE NAUTILUS. 87 



On the other hand, in nearly all the so-called smooth species there 

 is a distinct tendency toward the formation of a minute, transverse, 

 irregular and discontinuous rippling of the surface, which, especially 

 on the lateral slopes, tends to form irregular riblets extending anter- 

 iorly. The median portion of the anterior slope is less apt to be 

 affected in this way. 



While it is, as yet, too soon to speak positively, it seems possible 

 that an examination of a large amount of material may lead to dis- 

 carding the presence of a well-developed radial sculpture, unaccom- 

 panied by other peculiarities of size and shape, as a ground for spe- 

 cific distinction. The A. excentricus Morelet may be cited as an 

 example of this kind. It is described as smooth or with very fine 

 concentric lines only, and apparently on this ground alone is dis- 

 tinguished from the A. radiatus Guilding. Bourguignat (J. de C, 

 iv, p. 155), on other grounds, only allows it varietal rank ; but 

 Crosse & Fischer (Moll. Mex., ii, p. 37) hesitate to follow him on 

 account of the absence of the radiating striae, which are so prominent 

 in radiatus. An examination of specimens of A. excentrictis from 

 Texas, collected by Singley, shows that in all of them the radiating 

 sculpture is incipiently present and that in some there are well- 

 developed riblets present on the antero-lateral slopes, the median 

 portion of the anterior slope being practically smooth. (See also 

 Pilsbry, Naut., iii, p. 64.) A larger series would probably necessi- 

 tate tiie uniting of the two forms. 



The most striking feature, however, in the radial sculpture of the 

 Ancyli is the presence, in many of the species, of a circle of fine rib- 

 lets or striae on the apex, radiating from the apical scar. These 

 apparently have not been noticed before. When present in a species, 

 they are invariably to be found, and that quite independent of the 

 presence or absence of a radial sculpture over the entire surface, and 

 they may be entirely wanting in species with a well-developed ribbed 

 surface, as in A. eugraptus. When the radial sculpture is persistent 

 over the whole surface, the riblets originate from these apical striae, 

 but when tiiat sculpture is not present, they cease a very short dis- 

 tance from the apex. This apical sculpture is characteristic of the 

 more elevated species with an acute apex, of which A. rivularis is the 

 leading form. The scar in these species is situated on the tip of a 

 sharp, prominent apex and, with its circle of radiating ribs, is very 

 conspicuous and easily observed. 



