INVERTEBRATE PALAEONTOLOGY. 217 



Pholaaomya papyracea, m &h 



Plate 5, figs. 4, o, 6. 

 Pholadomya papyracea, Meek & Haydeu (1862), Proceed. Acad. Nat. Sci. Philad., XIV, 27. 



Shell under medium size, very thin and fragile, transversely subovate 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 regularly semi-ovate, 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 costse, 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 Agassiz 

 (particularly as illustrated by fig. 16, pi. 3, a, of his Etud. Cret.), than any 

 other form with which I have 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 Lieutenant Mullen of the United States Topographical Engineers. 



Locality and position. — Chippeway Point, near Fort Benton, on the Upper 

 Missouri ; in the Fort Benton group of the Cretaceous series of the North- 

 west. 



P li o I a <1 o in y si subveiitricosa, M. & H. 



Plate 39, figs. 8, a, b. 



Pholadomya subveiitricosa, Meek and Itayden (May, 1857), Proceed. Acad. Nat. Sci. Philad., IX, 142. 



Shell oval, very gibbous over the umbones and in the central and ante- 

 rior regions ; extremities rounded, the anterior end being slightly broader than 

 ■1 8 H 



