116 COLORADO FORMATION AND ITS INVERTEBRATE FAUNA, [bull. 106. 



PHOLADOMYID^l. 



Genus PHOLADOMYA Sowerby. 



Pholadomya papyracea M. & H. 



PI. xxvi, Fig. 1. 



Pholadomya papyracea Meek and ITaydcii, 1862 ; Proc. Acad. Nat. Sci. Phila., p. 27; 

 Meek, 1876, U. S. Geol. Sur. Terr., vol. ix, p. 217, PL 5, Figs. 4a and b. 



Revised description : 



" Shell under medium size, very thin and fragile, transversely sub- 

 ovate in outline, with length about once and a half the height, rather 

 compressed, the greatest convexity being in the anterior and umbonal 

 regions; posteriorly cuneate and a little gaping; outline of base reg- 

 ularly semiovate, its greatest prominence being a little in advance of 

 the middle; anterior side short and rounded ; posterior longer and more 

 narrowly rounded; hinge margin straight, not inflected so as to form a 

 defined false area, but subcarinate all along; beaks depressed, small, 

 and incurved, located near the anterior end of the valves, though not 

 terminal. Surface of each valve ornamented by about ten to twelve 

 small radiating costoe, which are crossed, and, as it were, cut into very 

 small tubercles by numerous, very regular, sharply defined, and much 

 more closely arranged concentric ribs and furrows, the markings being 

 all well defined on the internal cast. 



"Length about 1.16 inches; height, 0.76 inch; convexity, 0.55 inch. 



"In a side view this species more nearly resembles P. tenera of Agas- 

 siz (particularly as illustrated by Fig. 16, PI. 3a, of his l^tud. Grit.) 

 than any other form with which I Usbve compared it. It is decidedly 

 more compressed, however, as well as more depressed, and may also 

 be at once distinguished from that species by having no traces of a 

 false cardinal area, which is well defined in that species. The only 

 good specimen of it that I have seen was found by Lieut. Mullan of the 

 United States Topographical Engineers. 



u Locality and position. — Chippewa point, near Fort Benton, on the 

 upper Missouri, in the Fort Benton group of the Cretaceous series of 

 the Northwest. 7 ' 



Some specimens recently collected from the same horizon in northern 

 Montana by Mr. W. H. Weed are fully twice as large as the type. 



Pholadomya coloradoensis n. sp. 



PI. xxvi, Fig. 2. 



Shell transversely elongate oval, depressed; anterior end regularly 

 rounded from the beak to the base; basal margin forming an irregular 

 broadly rounded curve with the greatest convexity in the posterior 

 third; posterior end abruptly narrowed and subangular; beaks rather 



