INVERTEBRATE PALAEONTOLOGY. 257 



rior extremity ; surface ornamented by fine, regular, concentric striae, follow- 

 ing the flexures and curves of (lie free borders. 



Length, 0.40 inch ; height and convexity, each 26 inch. 



The concentric striae are most distinct near where they cross the linear 

 furrows descending from the beaks ; but diminish in size, become very closely 

 and regularly arranged, and very minutely crenate, as they curve upward 

 between this line and the margin of the hiatus. On being suddenly deflected 

 forward upon that portion of the valve over the hiatus, they as suddenly 

 become larger and more distinct. Of the two internal ridges, the anterior 

 one corresponding to the single furrow of the exterior, is the more sharply 

 defined. The posterior ridge is the larger, and leaves upon internal casts a 

 broad, rather rounded, deep furrow, extending obliquely from the back part 

 of each beak to the postero-basal margin. 



Traces, of apparently the pallial line seen on the cast, seem to indi- 

 cate a wide, shallow, nearly rectangular sinus, between the anterior and 

 oblique posterior internal -ridges. 



This species was named in honor of my lamented friend, the late Dr. 

 ^Vi]liam Stimpson. 



Locality and position. — Muscle-Shell River, Idaho Territory ; in the 

 Fort Pierre group, or formation No. 4 of the Upper Missouri Cretaceous 

 series. 



T u r ii u s ( X y 1 o p li a g c 1 1 a ) elegant ill us, M. & H 



Plate 30, figs. 10, a, b, c, d, e. 



Xylophaga elegantula,Meeli ami Haydeu (1H57), Proceed. Acad. Nat. Sci. Pbilad., IX, in. 

 Xylophagella elegantula, Meek (1864), Smithsonian Check-List N. Am. Cret. Fossils, lfi. 



Shell small, globose-cordate, with its closed posterior and postero-basal 

 margins rounded in outline; anterior side almost entirely occupied by the 

 very large rectangular hiatus, which extends more than one-third of the length 

 of the valves back from the front margin, and from the base above the 

 middle; umbones rather elevated and distinctly incurved, located a little in 

 advance of the middle. Surface ornamented by small, obscure, regularly- 

 arranged, concentric costse, and very fine lines of growth, which follow the 

 curves and angular flexures of the free borders; umbonal groove linear, and 

 nearly vertical. 



Length, height, and convexity, each about 0.19 inch. 

 33 H 



