166 PALEONTOLOGY. 



dinal and lateral leetli, i?o far as known, much as in the last-described species. 

 Surface ornamented by a series of very regular, distinctly-defined, and gener- 

 alh^ simple pli^^ations, which commence very small, and closely approxi- 

 mated along the dorsal margin just before the beaks, and after slight curves, 

 radiate and descend obliquely toward the posterior basal margin, increasing 

 in breadth and becoming more obtuse as they descend and diverge, and at 

 last in large specimens becoming obsolete before reaching the margins; 

 while another more or less similar series of plications sometimes originates 

 along the cardinal margins behind the beaks, and descends obliquely back- 

 ward and downward, so as to connect with those of the first-mentioned series 

 along the posterior umbonal slopes at very acute angles, somewhat like we 

 see on species of Goniomya. Marks of growth moderately distinct, becom- 

 ing sometimes stronger or subimbricating near the margins. 



Length of a large specimen, 2.75 inches; height, 1.50 inches; convex- 

 ity, about 0.85 inch. 



This beautiful species may be distinguished at a glance from the last 

 by its peculiar plications, the principal series of which, although originating, 

 as in that species, just before the beaks, always radiate obliquely backward 

 and downward across the lines of growth, instead of running horizontally 

 backward parallel to those lines. The fact that these plications do not con- 

 verge to the beaks will serve to distinguish' this shell from another associated 

 species, of which there are fragments in the collection, with a series of 

 very prominent plications converging more nearly to the apex of each 

 beak. In the latter, the beaks are also much more gibbous. 



In its style of ornamentation, this species seems to approach South 

 American types, such as U.liylea and U. Guaraniana, more nearly than any 

 of the numerous North American forms. Its radiating plications remind 

 one of the genus Castalia; but its form and hinge are entirely different, the 

 latter being that of true Unio. 



In Dr. Hayden's Second Annual Eeport of the Geological Survey of the 

 Territories, page 294, published in 1872, 1 proposed a subgenus Za-rop/cMnes, 

 for the reception of this species, which it can retain, if it shall be considered 

 desirable to separate it subgenerically or otherwise from the typical and 

 other established sections of the genus Unio, on account of its very peculiar 



