508 



CENOZOIC TIME. 



In most of the earliest Eocene species the brain was extremely small, 

 and especially the cerebral or anterior part ; that of the Dinoceras 

 might have been drawn entire through the cavity of the spinal cord. 

 This point is illustrated on Plate XI. (from Marsh), representing the 

 skulls reduced to a common length, and the brain cavity, of the Eocene, 

 Dinoceras (Fig. 3), the Miocene Brontotherium (Fig. 2), and the 

 modern Horse (Fig. 1). The Horse has a brain more than eight times 

 the bulk of that of the Dinoceras. See, also, Fig. 2, Plate VI. It is 

 seen in these figures that the posterior part of the brain, as Marsh 

 observes, has undergone little change of size, the enlargement having 

 been eminently in the cerebral portion. Cope also has recognized sim- 

 ilar facts. Moreover, Marsh has shown that the same principle is ex- 

 emplified in the fossil Birds, and also in the species of the Dinosaur 

 group of Reptiles. 



Characteristic Species. 

 Laramie Period. 



1. Radiates. — In the Lignitic coal-bearing bed, south of the Uintah Mountains, 

 Marsh found a Crinoid near the Marsupites of the Cretaceous, and in the same vicinity 

 Ostrea congesta, and scales of a Beryx, other Cretaceous forms. It is at present doubt- 

 ful whether the beds, are Cretaceous or Tertiary. 



2. Mollusks — In the coal-bearing or Lignitic group of the Rocky Mountain 

 region, referred by some to the Cretaceous formation, occur, at different levels (p. 501), 

 Inoceramus problematicus and other Inocerami, an Anchura, Gyrodesdepressa M., all Cre- 

 taceous forms. With these are found Cardium subcurtum M., Avicula (jastrodes M., Ostrea 

 soleniscus M., Cyrena CarltoniM., Modiola multilinigera M., Neritina pisum M., Tur- 

 ritella Coalvillensis M., T. spircncma~M., Cyprimera(?) isonema M., Eitlima funiculi M., 

 E. chrysalis M., E.(?)inconspicuaM., Melampus antiquus M., species of Unto; Corbicula 

 securis M., C. cequilateralis M., C fracta M., Viviparus trochiformis M. ; also, in some beds 

 iu the series, species of the fresh-water genera, Physa, Valvata, Cyrena, Neritina, with 

 those of Melampus, Eidima, Tumtella, etc.; or of Goniobasis, Viviparus, Corbicula, Cor- 

 bula, along with Ostrea, Anomia, and Modiola. In the fresh and brackish water Lignitic 

 beds of the Upper Missouri region, Figs. 908, 908 a, Corbula (Potamomya) mactriformis 

 M. & H. ; Fig. 909, Corbicula intermedia M. & H. ; Fig. 910, Unio priscus M. & H. ; 

 Fig. 911, Viviparus retusus M. & H. ; Fig. 912, Melania Nebrascensis M. & H. ; Fig. 

 913, Viviparus Leai M. & H. 



Meek states that the species of Melampus differ little from those of the Paris Tertiary 

 Basin ; that the species of Corbula, Corbicula, Physa, Cyrena, Neritina, are very similar 

 to species of the Lower Tertiary in the Upper Missouri region ; also that Vivijmrus tro- 

 chiformis M. is a Tertiary species of the Upper Missouri ; while, on the other hand, an 

 Anomia is very similar to a Texas Cretaceous species. 



In California, in the Tejon group, occur, according to Gabb, species of Ammonites 

 (one, A. jugalis Gabb), Fusus, Surcula, Typhis, Tritonium, Nassa, Pseudoliva, Olivella, 

 Fasciolaria, Mitra, Ficus, Natica, Lunatia, Neverita, Naticina, Scalaria, Terebra, Niso, 

 Cerithiopsis, Architectonica, Conus, Rimella, Cyprcea, Loxotrema, Turritella, Galerus, 

 Nerita, Margaritella, Gadus, Bulla; Solen, Corbula, Necera, Tellina, Donax, Venus, 

 Meretrix, Dosinia, Tapes, Cardium, Cardita, Lucina, Crassatella (C- alta Con.), Myti- 

 hos, Modiola, Avicula, Arci, Axincea, Pecten, Ostrea, with the coral Trochosmilia striata 

 Gabb. 



3. Fishes, Reptiles — In the beds of the Upper Missouri, occur scales of Lepi- 



