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University of California Publications in Geology 



[Vol. 8 



locality Cope also described M. renidens and M. suicide ns, species 

 which have since been regarded as belonging to the synonymy of 

 M. harlani. The Mylodon skull recorded by Cockerell 4 from near 

 Walsenburg, Colorado, has been identified by Allen as M. harlani. 

 The species Mylodon sodalis as originally described by Cope 5 was 

 established on several phalanges from the Fossil Lake beds of Oregon. 

 Subsequently Cope received an imperfect mandibular symphysis from 

 near the locality of the type and referred it to Mylodon sodalis. A 

 description of the specimen has not been published. 



In the excavations carried on by the University of California in 

 the asphalt deposits of Rancho La Brea at intervals between 1906 and 

 1912, skeletal material representing the genus Mylodon was found in 

 great abundance, in marked contrast to the meager representation of 

 Megalonyx and Nothrotherium. The splendid series of mylodont 

 skulls affords an unusual opportunity for the study of variation within 

 a group, and throws light upon the specific determination of other 

 mylodont material from Pleistocene deposits of North America. The 

 Rancho La Brea collection contains skulls of both the M. harlani and 

 P. vebrascensis type, but the range of variation in the series is such 

 that it seems necessary to include the latter form in M. harlani. No 

 skull material of mylodont forms clearly separable from M. harlani 

 has thus far been recognized in collections from the asphalt beds. 



In advance of a consideration of the entire edentate collection, 

 the following discussion of the skull and dentition of the Rancho La 

 Brea Mylodon is presented. 



For kind criticism I am greatly indebted to Professor John C. 

 Merriam, under whom the work on the ground-sloths of the asphalt 

 beds is being conducted. Comparison has been facilitated through 

 the courtesy of Mr. Barnum Brown and Dr. W. D. Matthew, who 

 have furnished photographs and measurements of the skull of P. 

 nebrascensis for use in comparative study. Dr. Glover M. Allen and 

 Professor T. D. A. Cockerell have also very kindly made several 

 measurements of their specimens at my request and have provided 

 outline drawings. To each of these gentlemen I desire to express my 

 sincere thanks. 



4 Cockerell, T. D. A., A Fossil Ground-Sloth in Colorado, Univ. Colo. Studies, 

 vol. 6, pp. 309-312, 2 pis., 1909. 



5 Cope, E. D., Descriptions of New Extinct Vertebrata from the Upper Ter- 

 tiary and Dakota Formations, Bull. U. S. Geol. Geogr. Surv. Terr., vol. 4, pp. 

 385-386, 1878. 



