TERTIARY AGE. 



501 



The above remarks on the animal life relate only to the Middle 

 Eocene and later species. The Lignitic beds, or Lower Eocene, of the 

 Rocky Mountain region and the Pacific Border are remarkable for 

 combining, along with species of a true Tertiary character, others that 

 are characteristically Cretaceous, owing to the fact that the Cretaceous 

 strata pass up without break or marked transition into the Lignitic 

 Tertiary. These Cretaceous and Cretaceous-like species include Ino- 



Fiffs. 908-913. 



Xamellibranchs. — Figs. 908. 908 a, Corbula (Potamomya) mactriformis ; 909, Uyrene (Corbicula) 

 intermedia; 910, Unio priscus. Gasteropods. — Fig. 911, Viviparus retusus ; 912, Melania Ne- 

 brascensis ; 913, Viviparug Leai. 



ceramus problematicus (Fig. 837, p. 461), and other allied species, which 



occur at various levels, through thousands of feet of rock, and are 



abundant in some beds. In Cali- 



f • a u x- ' Fi g s - 914-916. 



iornia, an Ammonite continues to 



the top of the Lignitic series. 

 Another peculiarity, already allu- 

 ded to, is the abundance of fresh- 

 water shells in some beds. Some 

 of these fresh-water species, from 

 the upper Missouri region, are 

 represented in Figs. 908 to 913. 



II. Vertebrates. — The Lig- 

 nitic or Lower Eocene beds have 

 not yet afforded any remains of 

 Mammals, and no Vertebrate re- 

 mains excepting those of Fishes 

 and Reptiles. Two Saurians occur 

 in it, related to the Dinosaurs ; and 

 this is another example of the 

 Cretaceous feature of the beds. 

 One specimen was found by Meek, 

 near Black Butte in Wyoming, Teetk op gi; 



and another, related to the Mega- dens ; 915, Lamnaelegans ; 916, Notidanus prim- 



losaur, by Marsh, south of the Uin- igenius ' 

 tah Mountains. 



