400 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



Fig. 54. 





Fig. 54. A septum of same from a smaller specimen, magnified about, one-third larger than the natural 

 size. — (After Hall and Meek.) 



In order to give the student belter means of comparing B. grandis with 

 B. ovatus than the figures of the form referred doubtfully to the former on 

 plate 83 afford, the preceding cuts from the original published figures of 

 B. grandis are added. 



Locality and position — The type-specimens of B. grandis were found in 

 the Bad Lands of Dakota, in the very upper beds of the Cretaceous, exposed 

 by ravines cut down through the Tertiary bone-beds. The specimen here 

 doubtfully referred to it, and figured on plate 3.'!. came from the Cheyenne 

 River, near the Black Hills, Dakota. The beds at both localities seem to 

 belong to the Fox Hills group of the Upper .Missouri Cretaceous series. 



I! :s <• ii B i l '. ■ «. compress us, Say. 

 Plate 20, figs. 3, a, b, c. 



Baciditcs compressus, Say (1821), Am. Jour. Sei. and Arts, II, 41. — Morton (18:34), Synop. Org. Remains 

 Cret. Group of (be U.S., 43, pi. ix, lig. 1 ; and Jour. Acad. Xat. Sci. Philad., VIII, -211.— Hall 

 and Meek ( 1^."4), Mem. Am. Aead. Arts and Sei.. Boston, V (n. B.),400, pi. v, fig. 2; and pi. vi, figs. 

 8 and 9.— Meek and Hayden (1860), Proceed. Acad. Nat. Sci. Philad., XII, 421.— Meek (1864), 

 Smithsoniau Check-List X T . Am. Cret. Fossils, 23. 



Shell attaining a large size, rather rapidly tapering, particularly in the 

 young, or near the smaller extremity of adult specimens, strongly compressed 

 laterally in medium-sized examples, but more convex in the young and 

 toward the larger extremity of large adults; non-septate portion of fully- 

 developed specimens, provided with large, broad, lateral undulations; lines ol 

 growth .generally obscure; siphonal margin sometimes crossed by small, 

 undefined wrinkles; transverse section, like the outline of the aperture, 

 varying with size and -age, being ovate in very small specimens, strongly 

 compressed in medium-sized examples, and proportionally more broadly 

 ovate in the large adult. 



