8 A Remarkable Ground Sloth 



the hinder part extending halfway up the sides of the muzzle. Viewed vertically, they 

 swing forward in a marked curve and are slender anteriorly, somewhat as in the Santa 

 Cruz genus Hafalofs from South America. As in Scott's figure of the latter, the tip of 

 the right premaxillary is slightly in advance of that of the left. Stock's description of 

 the muzzle of N. shastense is in entire agreement with our skull, and the same is true 

 of the palate, as far as can be seen because of the obscuring soft parts. The pterygoid 

 bullae have large foramina, agreeing with shastense but differing from the South 

 American N othrotherium described by Reinhardt. The glenoid surface of the skull 

 has its articulating cartilage preserved. This is decidedly concave longitudinally on 

 the articulating surface, thus inhibiting any fore and aft movement of the mandible. 

 The vomer has a peculiar asymmetrical, longitudinal ridge on its palatal aspect. This 

 starts on the left of mid-line anteriorly, curves widely to the right, then back to the left 

 again. This is shown in Stock's Figure 10 A but not mentioned in the description. 



(See Fig. 5.) 



The mandible in the Yale specimen shows no marked distinction from those of 



the Rancho la Brea. 



Symphyseal Ossicle— (Fig. 3, PI. Ill, A.) There is an ossicle of peculiar 

 structure lying in the spout-like symphysis which for convenience may be called the 

 "symphyseal ossicle." This bore against the premaxillary region of the palate and was 

 used in securing food, taking the place of the lower incisors of ruminants. This hith- 

 erto unknown element has been detached from its place at the mandibular symphysis 

 but is clearly revealed in situ in some of the photographs taken by Mr. Waterhouse, 

 so there is little question as to its position and orientation. From above it is a pointed 

 ellipse with a somewhat roughened surface. Below, it has a median longitudinal keel 

 dividing the highly rugose surface into two trough-like areas. Posteriorly the ossicle 

 turns down sharply. A broken surface shows a very cancellous structure. Shrunken 

 soft parts on the inner side of the left ramus show a rugose surface which is the recipro- 

 cal of part of the under side of the ossicle. 



The under surface of the left premaxillary bears a bit of thick, hard, roughened 

 cartilage, which, when fully complete, doubtless bore against the ossicle below. This 

 shows no trace of calcification, however; but if it did, the texture would possibly re- 

 semble that of the ossicle. 



Hyoid Arch (PI. Ill, A).— The right stylohyal is present in the Yale specimen. 

 It does not differ from Stock's description. It was in situ, attached to the styloid proc- 

 ess of the skull. 



MEASUREMENTS* 



Stock, Stylohyal No. 1540 

 Y.P.M. No. 13198 



* Note : The measurements are in millimeters throughout. 



