802 THE WHITE EIVER FAUNA. 



toid is grooved inwards and forwards to the inferior side of the skull. The 

 meatus auditorius is wide, and has no infeiior wall. There is an extensive 

 petrous fossa inclosed between the exoccipital behind, the basisphenoid and 

 basioccipital within, the alisphenoid in front, and the squamosal and mas- 

 toid externally. This is not overroofed on the inner side by a prolongation 

 of the lateral border of the basisphenoid, as in JErinaceus. In the middle of 

 the fossa is situated the petrous bone, which extends from the mastoid in- 

 wards and forwards to the adjacent parts of the basioccipital and basisphe- 

 noid. There is a well-marked postglenoid process, which is divided into 

 two parts by a deep notch. The inner portion is directed obUquely some- 

 what forwards. Just inside of it a crest, probably the inferior border of the 

 pterygoid process of the sphenoid, extends inwards and forwards, joining the 

 longitudinal pterygoid. The latter terminates at the inner border of the 

 large fossa above described. The squamosal bone has a remarkable exten- 

 sion behind the postglenoid process, and then rises into the oblique crest 

 which forms the boundary of the inion. Longitudinal crests connect this 

 with the superior extension of the postglenoid process, inclosing a fossa with 

 a subsquamosal foramen, a postglenoid foramen between them. In this re- 

 gion this genus is identical with Leptictis, and resembles Solenodon more than 

 any other living form. In that genus the mastoid is not larger, but is on 

 the lateral instead of the posterior face of the skull. The inferior part of 

 the squamosal in Mystomys also resembles considerably that of Mesodectes, 

 but the mastoid is far larger, and the otic region is roofed by the basisphe- 

 noid wings. The squamosal is much shortened behind in JErinaceus, and 

 there is no postglenoid process. The basioccipital and basisphenoid are 

 continuous, as in Solenodon. The subsquamosal foramen is especially char- 

 acteristic of the Didelphidce. 



The cervical vertebrce are short and transverse, and have well-developed 

 lateral arterial foramina and diapophyses. The centra are depressed to a 

 considerable degree, and are without hypapophyses. The neural arches are 

 narrow, and without spines. The atlas is expanded, and has a very short 

 diapophysis. The axis has a solid processus odontoideus. The dorsal verte- 

 brae are smaller than the cervical in transverse diameter of the centrum, 

 which somewhat exceeds the length ; the articular faces are nearly plane. 



