332 Bulletin American Museum of Natural History. [Vol. XXVII, 



omnivorous niodification of the cheek teeth are consistent Avith arboreal 

 habits, which are retained in some of the less specialized representatives of 

 the modern groups, e. g., Sciurus, Myoxus, Anomahtrus, Octodon, Hystrix. 

 As regards the soft anatomy the stem Rodents may be conceived to have 

 retained many primitive INIammalian characters in common with the Mar- 

 supials, Edentates and Insectivorcs, and some of these characters were trans- 

 mitted to various modern genera. The right and left uteri were entirely 

 distinct and a reduced cloaca may have been present {cf. Castor); the yolk 

 sack was large (in certain modern forms it even forms a small true placenta; 

 (vide supra); the brain was of low type, macrosmatic and with small, 

 smooth cerebra; traces of scales may have persisted, especially on the tail; 

 the eye also was probably of low type, for according to Lindsay Johnson 

 ophthalmoscopic examination of many modern Rodents reveals the following 

 among other primitive mammalian characters: crystalline lens appearing 

 as if formed of concentric circles; fundus usually of reddish-gray type; an 

 aster of radiating fibres present; choroid vessels often visible; small but 

 distinct pecten retained in Dasyprocta and others (cf. Marsupials). 



Conclusion. The known Lower Eocene Rodents had already acquired 

 the ordinal characters in the skull and dentition, and the Lower Oligocene 

 Lagomorphs were almost as well differentiated from the contemporary 

 Sciuromorphs as are the modern Hares from the Squirrels; so that the two 

 suborders could hardly have run together before the Basal Eocene; while 

 the point of union of the Rodent stem with any other mammalian order may 

 lie well back in the Cretaceous. 



The early hypertrophy of the incisors and the consequent modifications 

 of the skull and jaws marked but did not obliterate many primitive Placental 

 characters, and it seems not improbable that the stem forms of the Rodentia 

 were Mesozoic Placentals, allied to the ancestors of the modern Insectivora 

 and possibly to the contemporary ancestors of one or more of the Paratherian 

 or Edentate orders. 



IL The Edentate Orders. 



Outline History of the ordinal Classification. 



1G93. Ray groups "Echinus terrestris Tatou sive Armadillo, Talpa, 

 iNIusaraneus, Tamandua, Vespertilio, &" Ai sive Ignavus" [Bradypus] in a 

 division "Anomala" of the "Unguiculata." 



1758. Linnaeus includes in the order "Bruta" the genera Elephas, 

 Trichechus [ihmatus], Bradypus, Myrmecophaga, Manis. 



177L Thos. Pennant includes the following forms under "Digitated 



