70 MATTHEW— THE OSTEOLOGY OF SIXOPA. 



[April 13. 



skull, as in carnivora, while in marsupials and insectivores it has a 

 large exposure on the back of the skull. The brain is very small 

 and of inferior type, as in marsupials and all primitive mammals. 

 The occipital and sagittal crests are high, as in the carnivorous 

 marsupials. The tympanic bullae are not preserved and probably 

 were incompletely if at all ossified, and loosely attached to the 

 skull as in marsupials and insectivores. In modern carnivora they 

 are completely ossified and fast to the skull. But there is no trace 

 in Sinopa of the supporting plates from the alisphenoid and basi- 

 sphenoid bones around the margin of the bulla, the so-called 

 " false bulla," which is more or less developed in most insectivora 

 and marsupials. In Hyanodo'n the bullae are ossified to a varying 

 degree in the different species, in some apparently not at all, in 

 others a loosely attached bony ring, in others again a complete 

 osseous bulla ; but there is no trace of false bulla. 



The teeth resemble those of many carnivorous marsupials, the 

 molars being triangular with transverse and oblique shearing edges ; 

 but the dental formula is that of eutherian mammals, three incisors, 

 a canine, four premolars and three true molars, while the marsupials 

 have four or five incisors, canine, three premolars and four true 

 molars. The angle of the lower jaw is like that of typical carniv- 

 ora, and shows no trace of the marsupial inflection. This inflected 

 angle is seen quite as clearly in Cretaceous as in modern marsupials 

 and is evidently a distinction of very ancient origin. 



The details of construction of the skull, especially the basicranial 

 bones and foramina, agree entirely with the true carnivora, and 

 show that the marsupial resemblance is a superficial one. 



The vertebrae agree with carnivora in all important points. The 

 vertebral artery perforates the atlas and does not perforate the 

 seventh cervical. This condition prevails in carnivora and most 

 eutherians ; in marsupials as far as I have examined, the reverse is 

 the case. 



There are 13 dorsals and 7 lumbars, making a dorsolumbar 

 formula of twenty as in carnivora instead of nineteen as in marsupials. 

 The dorsolumbar formula is known in only a few creodonts. In 

 Oxyecna, and probably in Patriofelis and Hyeenodon, it was twenty 

 as in Sinopa; in Dromocyon nineteen according to Wortman. It is 

 probable that in all Oxyaenidae and Hyaenodontidae it was twenty 

 and in the Mesonychidae nineteen, this family approaching the 



