JUEASSIC FOSSILS. 365 



above. In the smaller individuals it is narrower, somewhat sloping from 

 above or obliquely truncate. Anterior end very slightly prolonged below 

 the middle of the height and excavated beneath the beaks. A narrow 

 linear escutcheon of considerable length is seen on many of the separated 

 valves. Anterior muscular impression rather large and strongly impressed, 

 as shown by the strong markings on internal casts; posterior impression 

 not distinguishable; pallial line faint, and the space within its limits often 

 marked by radiating lines, as in the recent forms of the genus. 



Surface of the valves ventricose, with a distinctly angular umbonal 

 slope in the larger specimen, but often obtusely rounded in the smaller 

 ones. A broad but rather faint depression runs obliquely backwards from 

 the beak to the middle of the basal border on many individuals, but is not 

 always present. Surface of the shell marked by irregular concentric lines 

 of growth parallel to the margin of the valve. 



The species is closely allied to a form described by Mr. Meek (in MS.) 

 under the name of Trepezium (Pachymya) truncata, but is proportionally 

 very much more elongated than the specimens which are so labeled in the 

 collection at the Smithsonian Institution. The shells have been exceedingly 

 abundant in certain layers, being densely packed together, so that it is im- 

 possible to separate the individuals. They are quite variable in form at 

 different stages of growth, the younger shells being transversely elongate- 

 ovate, narrowed and rounded, or very obliquely truncate from above 

 posteriorly, and becoming almost squarely truncate behind in the older 

 specimens, the slight sinuosity of the basal margin being hardly noticeable 

 until they attain to near their adult size. 



Formation and locality. — In a hard and somewhat silicious limestone of 

 reddish color, at a level of 350 feet above the Triassic beds east of the 

 Belle Fourche River, near Bear Lodge Butte, Black Hills. 



TEAPEZIUM SUBEQUALIS. 



Plate 5, figs. 5-8. 



Trapezium subequalis Wliitf., Prelim. Eept. Pal. Black Hills, 1877, p. 19. 



Shell small, transversely elongate, subelliptical, about twice and a half 

 as long as high, or nearly so. Valves somewhat ventricose, with large 



