1910.] Primitive Characters of the Marsupial Skull. 221 



occipital and basioccipital are sections of the transverse venous sinus. In 

 front of the periotic Ues the internal opening of the foramen ovale (\\). 

 In the floor of the basisphenoid we see the piiuitary fossa, without anterior 

 or posterior clinoid processes, and immediately external to it is the carotid 

 foramen. External to this, in turn, lies a longitudinal groove for the Gas- 

 serian ganglion (of the trigeminus) terminating anteriorly in the foramen 

 rotundum (Vj), which tunnels through the alisphenoid. Anteriorly to the 

 foramen rotundum and pituitary fossa is the sphenorbital fissure (for. lac. 

 ant., for nerves II, III, IV, Vj, VI); above and in front of this is the small 

 mesially placed orbitosphenoid, which in Didelphis is hollow and invaded 

 anteriorly by the posterior ethmoturbinals. Between the antero-lateral 

 border of the orbitosi:)henoid, the frontal, and the cribriform plate is the 

 ethmoid foramen (for the internal nasal nerve, a branch of VJ. 



A noteworthy feature of the internal view of the skull of Didelphis is that 

 on account of the imbricating relations of the edges of several bones the 

 external boundaries are widely removed from the internal boundaries. Thus 

 the parietals broadly overlap the frontals exteriorly so as to confine the latter 

 to the olfactory region ; but interiorly the frontals extend well backward so 

 as to cover the anterior and antero-superior portions of the cerebrum. The 

 alisphenoid which has a very extensive external distribution is limited 

 internally to a moderate anteroposterior wing. The squamosals, which are 

 so large in the external view of the Marsupial skull, in Didelphis are almost 

 excluded from the brain cavity, and represented only by a narrow strip in 

 front of the petrosal. The alisphenoids are fused with the basisphenoids 

 even in young skulls. 



As regards the ethmoturbinal complex Didelphis is seen to have a large 

 well-ossified mesethmoid, which is continuous posteriorly with the median 

 wall of the orbitosphenoid and ends anteriorly in a subvertical edge. The 

 mesethmoid is supported interiorly in the dorsal channel of the slender cpiill- 

 like vomer. The ethmoturbinal scrolls are limited to the middle and pos- 

 terior portions of the nasal cavity, the anterior part of which is occupied by 

 the well developed maxilloturbinals. 



Primitive Mammalian Characters of tlie Marsupial chondrocranium. 



An analysis of Parker's researches (1886) on the development of the 

 skull in ISIarsupials, Edentates and Insectivores, yields the following list of 

 characters, which indicates that the ^Marsupial chondrocranium, in its under- 

 lying features, represents a lower plane of mammalian evolution than does 

 that of primitive Placentals. 



(1) Boat-shaped chondrocranium with orbitosphenoid wing flush with 



