400 DESCRIPTIONS OF NEW SPECIES OF FOSSILS, 



terminal divisions are less distinctly bifid ; margins and extremities sinuate and digi- 

 tate. Ventral saddle as wide as the inferior lateral lobe, two thirds as high as the 

 lateral saddle, and less deeply divided at the top into two unequal parts, the right or 

 dorsal division being again divided into two unequal parts, the lower division of which 

 is somewhat bipartite. Ventral lobe narrow, about half as long as the ventral saddle, 

 digitate at the extremity, and deeply sinuate on the sides. Angle of the apex, as 

 deduced from the convergence of the dorsal and ventral margins by the measurement 

 of several specimens, 3i to 4. Longest diameter of largest specimens, 2.8 inches ; 

 shortest diameter of same, 1.7 inches. Shell on the ventrum, .13 inch thick; on the 

 dorsum, .1 inch ; and on the sides, about .05 inch thick. 



In a septate portion of a specimen, the cast gave, in its largest diameter, 1.4 inch ; 

 shortest diameter, .9 inch; diameter of siphuncle, .1 inch. 



Probable length of largest specimen in this collection, in its perfect state, 3^ feet. 



Locality and Position. Great Bend of the Missouri, and various other localities 

 on that river between Fort Pierre and the mouth of Big Sioux River. Fourth division, 

 ranging through its entire thickness. 



X 



BACULITES COMPRESSUS. 

 PLATE V. FIG. 2, a, I. PLATE VI. FIGS. 8, 9. 



B. compressus, SAY, Amer. Jour. Sci., Vol. II. p. 41. 

 " " MOBTON, Synopsis, 1834, p. 43, PI. IX. fig. 1. 



" " " Jour. Acad. Nat. Sci. Phil., Vol. VIII. p. 211, 1842. 



Shell elongate, extremely compressed, gradually tapering from the base ; section very 

 compressed ovate ; surface marked by lines of growth, which cross the dorsum, and, 

 bending obliquely downwards, curve outwards till they pass the centre of the side; 

 when they turn more abruptly outwards and again curve upwards, and cross the ven- 

 trum in a narrow arch. Lines of growth more prominent on the dorsum, forming 

 faint undulations across the surface. Septa symmetrical ; dorsal lobe very wide, and 

 little more than half as long as the dorsal saddle, deeply divided into two widely 

 separated branches, each of which is again divided into two unequal parts, which are 

 sharply and unequally digitate. Dorsal saddle twice as wide as the superior lateral 

 lobe, deeply divided by the acute, sharply digitate auxiliary lobe into two nearly equal 

 parts, each of which is subdivided into three branches, the left or ventral division 

 larger and more irregular than the other ; extremities of the branches obtuse. Supe- 

 rior lateral lobe narrow, one third longer than the dorsal lobe, divided into three 



