SKELETON OF ^LVKSUPIALIA. 347 



a coarse cellular diploe ; the frontal bones are about two and a 

 half lines thick. In the Ursine Dasyure the cranial bones have 

 a similar texture and relative thickness. In the Koala the tex- 

 ture of the cranial bones is denser, and their thickness varies from 

 two lines to half a line. In the Kangaroo the thickness varies 

 considerably in different parts of the skull, but the j^arietes are 

 generally so thin as to be diaphanous, which is the case with the 

 smaller ^Marsupials, as the Potoroos and Petaurists. The union 

 of the body of the second with that of the third cranial vertebra 

 takes place in the marsupial as in the placental Mammalia at the 

 sella turcica, which is overarched by the backward extension of 

 the orbitosphenoids. The optic foramina and the fissura3 lacera3 

 anteriores are all blended together, so that a wide opening leads 

 outward from each side of the sella. Immediately posterior and 

 external to this opening are the foramina rotunda, from each of 

 which in the Kangaroo a remarkable groove leads to the fossa 

 Gasseriana at the commencement of the foramen ovale ; the same 

 groove is indicated in a slight degree in the Dasyures and Pha- 

 langers, but is almost obsolete in the Wombat and Koala. The 

 entocarotid canals pierce the basisphenoid, as in Birds, and ter- 

 minate in the cranial cavity very close together behind the sella 

 turcica, which is not bounded by a posterior clinoid process. The 

 sphenoidal bulla, which forms the chief part of the tympanic 

 cavity in the Perameles lagofis, forms a large convex protuberance 

 on each side of the floor of the cranial cavity in that species. 

 The petrosal in the Kangaroo, Koala, and Phalangers, is im- 

 pressed above the meatus auditorius by a deep, smooth, round 

 pit, which lodges the lateral appendage of the cerebellum, as in 

 Birds. The corresponding pit is shallower in the Dasyuri, and 

 is almost obsolete in the AVombat. The middle and posterior 

 fissurae lacer^e have the usual relative position, but the latter are 

 small. The condyles are each perforated anteriorly by two fora- 

 mina in most of the Marsupials, the Thylacinus forming the ex- 

 ception and showing only one. The foramen magnum is of great 

 size in relation to the capacity of the cranium ; the aspect of its 

 plane is backAvard and slightly do^^Tiward. A venous canal leads 

 from the lateral sinus between the upper part of the petrosal and 

 the squamosal, and perforates the latter behind or above the root 

 of the zygoma. 



In the Kangaroo and Phalanger a thin ridge of bone extends 

 for tlie distance of one or two lines into the periphery of the 

 tentorial process of the dura mater, and two sharp spines are sent 

 down into it from the upper part of the cranium in the Phalan- 



