422 PATAGONIAN EXPEDITIONS I PALAEONTOLOGY. 



The periotic has a large exposure upon the occipital surface, sending up 

 a tongue-like process between the squamosal and exoccipital, resembling 

 the arrangement seen in Dolichotis rather than that in the Erethizontince. 

 The supramastoid process of the squamosal is quite narrow, and between 

 this process and the periotic is a deep groove, which is even more pro- 

 nounced than in Dolichotis. The frontal zone is elongate and the fore- 

 head flat, without protuberances formed by the sinuses ; the supraorbital 

 ridges are decidedly more prominent than in Erethizon, and very short 

 postorbital processes are present in some species, not in others. The 

 dorsal ramus of the maxillary zygomatic process is broad, less compressed 

 antero-posteriorly than in Erethizon and the lachrymal, though small, is 

 considerably larger than in the latter. The proportions of the rostrum 

 and nasals differ much in the various species. The hard palate is of 

 nearly uniform width between the teeth ; the posterior nares are narrow 

 and V-shaped. 



Known mandibles are all incomplete, lacking the condyle and angle ; 

 the masseteric crest is even more prominent than in Steiromys and the 

 masseteric fossa deeper ; the pterygoid crest also, as far as it is preserved, 

 is better developed than in the last-named genus. 



Considerable parts of the skeleton are associated with an individual of 

 S. varians (No. 15,168) and with one of S. latidens (No. 15,944). All 

 the bones are very small, slight and delicate. Compared with Erethizon, 

 the differences are so many and so marked as to indicate a different man- 

 ner of life ; probably these animals were terrestrial rather than arboreal in 

 habits. 



As a whole, the neck is relatively a little longer than in Erethizon. 

 The atlas has a remarkably large, subcircular neural canal, which propor- 

 tionately far exceeds that in the recent genus, and the transverse processes 

 are thinner and more prominent than in the latter. In the axis the cen- 

 trum is broad and depressed and has a prominent ventral keel, and the 

 the neural canal is very large. The succeeding cervicals are short and 

 broad and all appear to have been without neural spines, a difference from 

 Erethizon, while the transverse processes resemble those of the latter in 

 their rod-like shape and in the very small development of the inferior 

 lamella. The anterior thoracic vertebrae are very small and have very 

 slender spines, which, though incomplete, appear to have been longer than 

 in Erethizon. The sacrum probably consisted of four vertebrae, three of 



