388 MAMMALIA. 



vacuities, while the skull and most parts of the skeleton closely 

 resemble the corresponding bones in the typical Creodonta. 

 The incisors are always small, but there are sometimes four 

 pairs of these teeth ; the canines are well developed ; and 

 the seven following teeth are more or less suggestive of the 

 dentition of the Tasmanian Thylacinus. 



Proth.ylacin.us (fig. 218). In this genus there are four pairs of incisors 

 above and three pairs below ; and there are seven teeth in each jaw behind 

 the enlarged canine. In the upper jaw the three foremost teeth of the 

 series are clearly to be regarded as premolars ; while the four teeth 

 behind are triangular in form and tritubercular, and there may be a 

 difference of opinion as to whether the first of these is to be termed pm. 4 

 or m. 1. The dental series, indeed, bears much general resemblance to 



FIG. 218. 



Prothylacinus patagonicus ; left mandibular ramus, outer aspect, one-half nat. 

 size. Santa Cruz Formation ; Patagonia. (After Ameghino.) 



that of the upper jaw of Stypolophus (fig. 214 6, p. 383), which is usually 

 interpreted as comprising pm. 4, m. 3. Of the seven teeth in the mandible 

 (fig. 218) the foremost three are simple conical premolars, each with a 

 cleft root ; the fourth is a comparatively small tooth much resembling the 

 last lower milk-molar of Hyasnodon; the remaining three teeth are 

 obviously true molars. The two rami of the mandible are fused together 

 at the symphysis. The humerus exhibits an entepicoudylar foramen. 

 The manus comprises four complete digits, with pointed ungual phalanges, 

 but digit no. I is represented only by its metacarpal. The genus Prothy- 

 lacinus occurs in the Santa Cruz Formation of Patagonia, and the length 

 of the mandible in the type species, P. patagonicus (fig. 218), is about 

 0-15 m. 



Borhysena has only three pairs of incisors above and below, and the 

 jaws are more robust than those of Prothylacinus, though the dental series 

 behind the canine is very similar in the two genera. The canine and the 



