104 University of California Publications in Geology [Vol. 10 



characters differing from those of Arctotherium; the true bears, and 

 such other carnivores as would be expected to occur in this formation. 

 The atlas is much larger than that of an Arctotherium specimen from 

 Potter Creek Cave, and differs from it structurally in several particu- 

 lars. The lateral wings are broken, but seem to have been broad and 

 to have extended with a wide sweep posteriorly somewhat as in 

 Ursus. The posterior portion of the lateral wings extends behind the 

 facets for articulation with the axis, as in Smilodon, without an inter- 

 ruption or notch such as appears in Arctotherium, Ursus, the dogs, 

 and the true cats. In the specimen found near no. 22362 the extension 

 of the wings behind the axis facets is buttressed by a high superior 

 ridge which runs across the region in which the notch would be sit- 

 uated in the true bears or in Arctotherium. In this particular this 

 atlas is again similar to that of Smilodon. One of the most important 

 characters is found in the position of the vertebrarterial foramen, 

 which in the Pliocene specimen is situated low down at the posterior 

 border of the lateral wings, and immediately behind the facets for 

 the axis. This situation is much as in Smilodon. In Ursus the 

 posterior opening of this foramen is situated high up on the posterior 

 side of the lateral wings. In Arctotherium it is situated considerably 

 in advance of the postero-superior border of the wings. The position 

 of this foramen in Arctotherium resembles more closely that of the 

 dogs than in the case of Ursus. 



The atlas from the Rattlesnake differs from that of the cats and 

 the clogs in that the passage for the vertebral artery through the 

 anterior side of the lateral wings is in an enclosed foramen instead 

 of in a groove or notch in the anterior border. Tn this respect this 

 atlas resembles that of Ursus and Arctotherium. 



The characters of the atlas found near no. 22362 resemble those 

 of the bears in the nature of the anterior portion of the vertebrarterial 

 passage, resemble the cats in the nature of the posterior extremity of 

 this canal, and resemble Smilodon in the form of the posterior border 

 of the lateral wings. It is interesting to note that, whereas the atlas 

 of Arctotherium is in certain respects more dog-like than that of 

 Ursus, the Rattlesnake specimen differs widely from the dogs in the 

 particular characters in which Arctotherium approaches them, and in 

 these characters the Rattlesnake form resembles Smilodon more closely. 



