72 MATTHEW— THE OSTEOLOGY OF SINOPA. [April 13, 



Hygenodont phylum. The genera Sinopa, Prototomus, Cynohycen- 

 odon, Pterodon and Hyccnodon show a series of stages in the develop- 

 ment of a highly specialized sectorial dentition, and with some 

 exceptions, in the specialization of the skull and skeleton so far as 

 they are known. The geological occurrence of the known species 

 of these genera precludes their being regarded as in the direct line 

 of phyletic descent. Sinopa and Prototomus are found together in 

 the Lower and Middle Eocene, while Cynohyoznodon, Pterodon and 

 Hycenodon occur together in the Oligocene. But without doubt 

 the genera represent very closely the stages through which the 

 phylum passed in its evolution, and that is about as much as it is 

 safe to assert of most phylogenetic series. 



The relationship of Sinopa to the Oxyaenidae, especially to 

 Limnocyon, is not yet clear. There is a great deal of resemblance 

 in skeletal characters, a marked diversity in the more significant 

 features of the skull. Most of the resemblance, perhaps all, is to 

 be explained as due to retention of primitive creodont characters, 

 but some may indicate a nearer relationship of Hyasnodonts to 

 Oxyaanids than to any other creodont family. 



