Occasional Papers of the Museum of Zoology 15 



ever, on one I still recognize vertical, shallow furrows, which 

 are about a line apart and extend almost an inch from the 

 beaks, which must give an individual character to the young, 

 uninjured shell, that helps to differentiate this species from 

 others"' (translation). The locality of the species is indefinite 

 (Mexico, legit cl. Liebmann), but it is probably a more south- 

 ern form than this subspecies. U . sphenorJiynchus C. and F. 

 (1894) has the same dimensions as the subspecies, but is very 

 sinuate ventrad ; has a much more definitely marked posterior 

 ridge ; the beaks are placed more posteriad ; and the posterior 

 tooth in the right valve is more trigonal than compressed. C. 

 and F. (1894) also give a figure (Ixv, 4) of what they con- 

 sider an aberrant shell of their U. tehuantcpccensls; it has 

 every appearance of my shell, plications and all. 



This species is apparently quite closely related to E. plexus 

 (Con.) and its identification is further confused by the very 

 great resemblance to what I think to be the young shells of 

 A. walkcri (see below). These latter are intermediate in shape 

 between £. plexus and E. licbuianni, and their lateral teeth 

 somewhat resemble those of the former. However, their right 

 pseudocardinals are more trigonal and they lack the vertical 

 furrows, typical of both species, although they possess some- 

 what similar, curved, posterior plications. 



Two quite well-marked lines of variation are present in the 

 lot. One is represented by figs. 20 and 21, and by two other 

 shells not figured. These four shells are much smoother and 

 more polished than are the rest (21 has no sign of ornamen- 

 tation), have a more strongly curved, dorsal line, and shorter 

 and more curved laterals, and tend to be somewhat higher. 

 The last two of these characters make them approach, in 

 appearance, the young shells of ^. walker i and E. plexus. The 

 other aberrant type is represented by a single left valve (fig. 



