6 A Remarkable Ground Sloth 



and sternal ribs, and certain epiphyses. The Rancho la Brea has yielded sufficient ma- 

 terial of this rare genus to permit the mounting of a composite specimen of remarkable 

 accuracy. But it is made up of several individuals, and the bones do not always show 

 the correct proportions, whereas the Yale specimen is a single individual, which elimi- 

 nates this source of error. Judging from proportionate measurements, ours represents 

 an animal about nine-tenths grown. Mr. Stock is to be congratulated, not only for the 

 splendid mount in the Los Angeles Museum, but also for his highly accurate interpre- 

 tation of the several elements ( 1913, 1925). His work is so detailed that a comparative 

 study of the present specimen is all that is necessary. 



MORPHOLOGY 



ENDOSKELETON 



AXIAL SKELETON 



The Skull (PI. Ill ; Text Figs. 3-5).— The skull and jaws are well preserved on 

 the right side but on the left are somewhat decayed. A portion of the hide 240 mm. 

 long covers the nasals, frontals, part of the parietals and maxilla of the right side, thus 

 concealing the mid-line sutures and full length of the nasals. On the right side of the 

 muzzle are vague traces of hair. The palate also bears some of the soft parts, as do the 

 premaxillaries and turbinals. A portion of muscular tissue is also present between the 

 skull and the ascending ramus of the jaw on the left. 



As the table of measurements (page 9) shows, the Yale skull is smaller than 

 any of the several N othrotherium skulls described by Stock in his memoir. This may 

 be due to the relative immaturity of the present specimen, as it is true of almost every 

 comparable measurement of the skeleton. The Yale skull varies, however, in its rela- 

 tive slenderness and comparatively high occiput and length of palate. To what ex- 

 tent these are juvenile characters is not evident. The ratios of length to breadth across 

 the cranium are for N othrotherium texanum type U.S.N.M. No. 8353, 285 to 100; 

 N. shasteme Rancho la Brea, average of six skulls, 300 to 100; Yale skull, 330 to 100. 

 In profile the Yale skull is decidedly more arched at the vertex and less flaring at the 

 muzzle, so that the facial contour is markedly different. In the latter detail there is 

 much variation within the Rancho la Brea group, and the arching of the nasals also 

 varies. These details of contrast, therefore, may well fall within the range of indi- 

 vidual variation or be due to age or sex and hence of no value for specific diagnosis. 

 The lachrymal is prominent, viewed from above, and the lachrymal canal is large. 

 The postorbital process of the frontal is almost obsolete. There is a marked depression 

 of the frontals above these processes. This feature also varies within the Rancho la 

 Brea group, being most marked in the largest individuals and practically absent in 

 the type of Stock's abandoned species of N. gracilicep, Los Angeles Collection Nos. 

 1800-13. There is no trace of temporal ridges in our specimen, and a sagittal crest is 

 only slightly indicated. There is a sag or depression in the median profile of the an- 



