1 8 University of Michigan 



The small river form (Arroyo Hueyapam) approaches quite 

 closely A. umbrosa (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 difiference 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 oi A. umbrosa and A. explicata 

 are very dissimilar, these two lots of shells show approaches 

 to both species, and it seems probable that umbrosa 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- 



