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. sphenorhynchus 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 I. (1894) also give a figure (lxv, 4) of what they con- 
sider an aberrant shell of their U. tehuantepecensis; 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. walkeri (see below). These latter are intermediate in shape 
between &. plerus and &. liebmanni, 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 4. walkeri and E. plexus. The 
other aberrant type is represented by a single left valve (fig. 
