1914] 



Stock: Mylodont Sloths of Rancho La Brea 



327 



DENTITION 



On the basis of the superior dentition the Rancho La Brea series of 

 skulls may be divided into the following groups : 



1. Skulls with five teeth on each side of the palate. 



2. Skulls with five teeth on one side and four teeth on the opposite 

 side of the palate. 



3. Skulls with four teeth on each side of the palate. 



As has been indicated in an earlier publication, these groups seem 

 to intergrade and they are now held to represent a single specific form. 



The superior tooth rows in most specimens do not diverge anter- 

 iorly as much as in Mylodon robustus. In no. 21159 the divergence 

 of the tooth rows approximates closely that in a skull of M. robustus, 

 considered by Lydekker 7 as belonging to a female individual. 



The first superior tooth, when present on both sides of the palate, 

 is either circular or oval in cross-section and is recurved as in M. 

 garmani. The extent to which this tooth may project from the alveolus 

 varies considerably. In group two the first tooth may be present on 

 either side of the palate and may vary in size without respect to the 

 age of the individual. In no. 21158 the alveolus shows the tooth to 

 have been quite diminutive. In group three the first tooth is absent 

 on both sides of the palate as in Paramylodon nebrascensis. 



The second superior tooth (first molar of P. nebrascensis) is 

 elliptical in cross-section and may vary somewhat in antero-posterior 

 diameter. In M. robustus the second tooth is not much larger than 

 the first and is not elongated antero-posteriorly as in the North 

 American species. In no. 21157 the right tooth is extremely elongated 

 antero-posteriorly and has a deep median sulcus on the outer surface. 

 The second tooth is recurved as in Brown's specimen and in M. gar- 

 mani. The beveling of the anterior side of the triturating surface 

 may extend backward to the middle of that surface. The distance 

 between the first and second teeth is not as great in M. harlani as 

 in M. robustus. 



The third superior tooth (second molar of P. nebrascensis) does not 

 differ essentially from Brown's description. The anterior side of the 

 tooth may occasionally show a faint groove and the sulcus on the 

 outer posterior face may vary in depth. 



« Stock, C, The Systematic Position of the Mylodont Sloths from Rancho La 

 Brea, Science (n.s.), vol. 39, pp. 761-763, 1914. 



i Lydekker, R., Contributions to a Knowledge of the Fossil Vertebrates of 

 Argentina, Part 2, Extinct Edentates of Argentina, Anales del Museo de La 

 Plata, Paleontologia Argentina, part 3, p. 79, pi. 49, fig. 2, 1894. 



