18 University of Michigan 
The small river form (Arroyo Hueyapam) approaches quite 
closely A. wmbrosa (Lea), as it has the more wedge-shaped 
form (figs. 44, 45), but the specimens are quite light-colored, 
while most of those from the Rio Medellin, like so many spe- 
cies from that river, are quite dark. Some of the adult 
males(?) from the Rio San Juan (fig. 46), on the other hand, 
quite closely approximate U. alienigenus C. and F. or even 
A. explicata. The females(?) from the Rio San Juan (fig. 
47) are not so rectangular as the males and are somewhat 
swollen along the posterior shoulder down to the posterior 
ventral margin, very much as in A. sapotalensis, although to 
a lesser degree. The young males of the last species are prac- 
tically identical in shape with those of this form, but may be 
easily separated by the difference in the pseudocardinals. The 
juvenile specimens from the Arroyo Hueyapam (fig. 43) are 
subrhomboid, and are beautifully rayed with green. ‘They 
have no sign of a dorsal “wing.” The pseudocardinals of the 
right valve are always oblique and almost parallel, but the size 
of the upper tooth is variable and the development of either 
appears dependent on the age of the individual. In the juve- 
nile specimens, they are lamellar, while in the older specimens, 
although always distinctly compressed, they are often quite 
heavy and jagged. The nacre of the adults is usually white, 
but may be tinged with either salmon or violet. 
Although typical specimens of 4. wmbrosa and A. explicata 
are very dissimilar, these two lots of shells show approaches 
to both species, and it seems probable that wmbrosa is a 
dwarfed, small-river, northern form (type apparently a female) 
of the same species of which explicata is the larger, southern, 
form (type apparently a male). U. alienigenus is an interme- 
diate form from the Goatzcoalcos River system. Strictly 
speaking, A. umbrosa (Lea) should have the priority, as More- 
