THE OSTEOL OGY OF SINOPA—MA TTHE W. 231 



a considerable degree of progress from the primitive carnivore type 

 toward the line which terminated in the large, highly specialized 

 Hyamodons of the American oligocene. As has already been observed, 

 the geological occurrence of the species of this phylogenetic line makes 

 it improbable that Ilyeenodon was directly descended from any of the 

 Middle Eocene species of Sinopa; it is more probably derived from 

 a Lower Eocene or earlier species. Wortman has suggested S. opis- 

 tliotoma as a possible ancestor, but this species does not entirely meet 

 the required conditions. 



The relationship to the carnivorous marsupials appears to be a remote 

 one, despite a considerable degree of superficial resemblance, due 

 chiefly to the retention of the primitive mammalian characters. In 

 all marsupials the angle of the lower jaw is inflected, the molars num- 

 ber four, the premolars not more than three, the basicranial region is 

 short and the mastoid exposure posterior and of large size, the carotid 

 canal pierces the basisphenoid, the pterygoid processes of the alisphe- 

 noid and palatine are little developed, and more or less of a false bulla 

 is formed; the dorsolumbar formula is 19, and there are numerous less 

 important details of form and structure in the bones, showing that the}'^ 

 are far removed from Sinopa or from any of the Creodonta. The 

 dorsolumbar formula oi Mesonyx^ according to Wortman, was 19, as in 

 the marsupials, and this genus also has a broad posterior expansion of 

 the nasals; but in the much more important characters of the base and 

 back of the skull, as in all other features of the skeleton, it is evidently 

 of true carnivore afiinities, somewhat disguised by a high degree of 

 specialization in certain parts. In Sinoj>a^ in Oxymia and Hyxnodon^ 

 and probably in Pdirlofelis, the only other creodonts in which the 

 dorsolumbar formula is known, it is twenty, as in all Carnivora, and 

 these genera have all evidently descended from primitive carnivore 

 ancestors, whose principal distinctions are given above. 



The Insectivora appear to be in many respects intermediate between 

 Carnivora and marsupials, but how far they are actually so would be 

 difiicult to say. It is clear that the Creodonts are not nearly related 

 to any living Insectivora, but we know so little about the past history 

 of the insectivore group that we can not yet say whether it is really a 

 homogeneous one or an arbitrary association of unrelated types. In 

 the features of the base and back of the skull they differ very consid- 

 erably from Carnivora and agree more or less with marsupials (the 

 course of the carotid canal differs from either group). They have the 

 Eutherian dental formula, a non-inflected angle of peculiar type in 

 the lower jaw, etc.; these features characterize the most primitive 

 and ancient known types as well as the modern ones. 



The position and relationships of the Eocene Carnivora have been 

 variously estimated b}^ the different writers who have studied and 

 described their remains. The incomplete specimens first found in the 



