Tertiary Vertebrata of the West. 



513 



with closely-packed cylindrical pleurondont teeth, having compressed 

 crowns with three cusps, the median one large and those at the sides 

 small. Diacium quinquipedale is founded on a sacral vertebra which 

 has no trace of a neural spine ; and indicates an animal as large as 

 any of the living Iguanidas. The genus Platyrhachis is only known 

 from vertebras which have the centra much impressed, the neural 

 spine reduced to a low keel, and the ridge connecting the zygapo- 

 physes deeply notched, a character which distinguishes these vertebras 

 from those of Peltosavrus : three species are known. Cremastosaurus 

 carinicollis is a Lizard known from cervical vertebras, somewhat like 

 those of Phrynosoma comutum, but with relatively small cup and 

 ball articulations, and a strong rib-like hypapophysis. 



There are four genera of Snakes founded on vertebras, which are 

 the only parts hitherto discovered. Aphelophis is very like the living 

 genus Charina, though the zygosphene is wider. Ogmophis, known 

 from two species, is an allied type, distinguished by the ridge which 

 extends from the parapophysis, and the groove which lies between 

 that ridge and the middle line of the centrum. Calamagras is a 

 similar type, but intermediate between the two last named. Neuro- 

 dromicus has vertebras which resemble those of one of the Crotalidas, 

 but has no process below the prezygapophysis, and has the hypapo- 

 physis less robust. 



The Mammalia of the White River period are not less numerous 

 than those of the Bridger and Wasatch rocks, but a great change 

 has come over the fauna. All the Amblypoda have disappeared, 

 and there are no representatives of the Tasniodonta, Tillodonta, or of 

 the Lophiodonts ; while the Creodonta and Mesodonta are rare. No 

 species of mammal found in these rocks is common to the under- 

 lying Eocene series. The Miocene species reach a larger average 

 size, particularly among the Perissodactyla and Carnivora. Life 

 would appear to have been abundant : for the author states that 

 many of the species are represented by great droves, and their bones 



c * — ■"• cf 



Peratherium fugax. 



form beds of considerable extent. The Marsupialia is represented 

 by doubtful types, some of which have already been referred to the 

 Creodonta, and are only placed with the Marsupials provisionally. 

 Peratherium, though represented by six species, is only known from 

 the cranium and mandibles. An excellent discussion is given of its 

 dental characters, with the result that the teeth differ from those of 



DECADE III. — VOL. III. — NO. XI 33 



