MAMMALIA. 



29 



Guinea-pig, Porcupine, Hare and Capybara, all have perfect 

 clavicles. The skeleton is slight and feeble. The Beaver and 

 Capybara are now the giants of the order ; but the Muridce are 

 the typical family. 



No unequivocal evidence has been obtained of remains of 

 rodents in strata more ancient than the Eocene Tertiary, but they 

 are frequent in the lowest fresh-water Eocene, belonging to the 

 Squirrel family. The hares appear in the Middle Miocene. In 

 the Pliocene most species belong to existing genera. The fossils 

 are chieflv found in lacustrine marls and bone-caves. 



No. 45. [21] Castoroides Ohioensis, Foster. 



Skull and Lower Jaw, right 

 RAMUS (cast). This species is the 

 most gigantic member of tlie or- 

 der of rodents hitherto discovered, 

 whetlier. recent or fossil. It is aliiu 

 to the Beaver, but differs chiefly in a 

 less development of the cerebrum, 

 in more prominent though more 

 slender, zygomatic arches, and in its 

 dentition. The incisors are fluted, 

 and the molars (numbering four in each ramus) consist of a series of elongated 

 elliptical plates of enamel which include the dentine. The Castoroides differs 

 from all other rodents in the size and conformation of the pterygoid processes 

 and fossae. All the processes and fossae of the lower jaw are remarkably de- 

 veloped. The genus abounded in North America throughout the Post-tertiary. 

 The original specimen, supposed to belong to an animal nearly six feet in 

 length, was found in 1841, in the Montezuma Marsh, near Clyde, N. Y., with 

 shells of existing species, and is preserved in the Cabinet of Geneva Col- 

 lege, N. Y. Size, 10 x 7. 



ORDER EDENTATA. 



The Edentates, the lowest of the placental mammals, are dis- 

 tinguished by the inferior character of their teeth, which have 

 no complete roots, no true enamel, and are usually monophydont 

 (without successors). There are no incisors,. with the exception 

 that in the Armadillo tlie front pair of upper teeth encroach 

 upon the intermaxillary bones, and assume the position of lateral 

 incisors. Two genei'a have no teeth at all, the Ant-eater {Myr- 

 mecophaga) and the Pangolin {Manis), while the Armadillos 



