FROM THE CRETACEOUS FORMATION OF NEBRASKA. 403 



lobes of the septa are much deeper, narrower, and less divergent in their branches ; 

 the digitations are sharper and more directly pointed in the longitudinal direction of 

 the shell. The auxiliary lobe of the ventral saddle is longer in this species, while the 

 extremities of the terminal branches are less deeply bifid than in B. ovattis. In this 

 species the two terminal branches of the superior lateral lobe are simply digitate, while 

 in B. ovafus they are deeply bifid, with obtuse sinuosities. Externally in its undula- 

 tions on the sides, this species resembles B. anceps of Lamarck, but will be readily 

 distinguished by the absence of a dorsal carina, and by its much deeper lobes with less 

 divergent divisions. A comparison of the details of the divisions of the lobes and 

 saddles shows a constant difference in the two species. 



Locality and Position. — Mauvaises Terres, head of Bear Creek. Fifth or upper 

 di\'ision of the section, and but a few feet below the base of the Titanotherium bed of 

 the Tertiary formation. From this point it is known to extend downwards some 

 twenty or thirty feet, and probably ranges through all the beds of the fifth division. 



The occurz-ence of this fossil at this locality indicates verv distinctly the line of 

 demarcation between the Cretaceous and Tertiary formations of this region ; and from 

 the absence of other fossils in this division, as far as known, no well-defined line has 

 heretofore been drawn between the deposits of the two periods ; and no connection 

 has hitherto been shown between the Eocene formation, containing Mammalian re- 

 mains, with the Cretaceous formation below. 



In examining this collection of specimens, we found no difficulty at the outset in 

 distinguishing the Bacidites ovatus and B. compressus in many large and medium 

 sized shells. At the same time, numerous smaller specimens presented a structvne so 

 different, that we were inclined to refer them to distinct species, until a further exami- 

 nation of specimens still more minute satisfied us that they were all to be referred to 

 the one species of B. ovatus, showing different degrees of development dependent upon 

 age and growth. A careful examination under a magnifier of a specimen only one 

 twentieth of an inch in diameter, showed the septa, which are so complicated in the 

 mature specimen, to be extremely simple, the lobes and saddles represented by simple 

 undulations. The two extremities of the same specimen also showed different degrees 

 of development, as seen in Plate YI. fig. 1, a, and 1, J, which are from the smaller and 

 larger extremities respectively of the specimen, fig. 1. An individual of larger growth 

 (about .1 inch diameter) shows a still further advance in the development of the 

 lobes and saddles, with their principal divisions, as shown in fig. 2, 2, a. A still fur- 

 ther advance is shown in another individual of .16 inch diameter, in fig. 3, 3, a, while 



