218 Bulletin American Museum of Natural History. [Vol. XXVII, 



characters of the skull. The following comparison of a smaller insectivo- 

 rous Didelphid {Marmosa simonsi) with a larger more specialized, more 

 carnivorous form (Didelphis virginianus) will serve to illustrate how many 

 differences may result merely from an increase in size and a change from 

 insectivorous to semi- carnivorous habits. 



The skull of Marmosa (Fig. 16) agrees with those of the more primitive 

 Placental Insectivora {e. g., MicrogaJe, Idops) in its minute size, broadly 

 triangular contour as seen in top view, rather short face, pointed muzzle, 

 relatively large rounded brain case without sagittal or occipital crests, 

 incisors row anteroposterior rather than transverse, canines small, premolars 

 pointed, upper molars sharp-cusped, triangular, and lower molars tuberculo- 



sectorial. 



It is reasonably certain that these characters, especially the relatively 

 laro-e rounded brain case, are directly dependent upon minute size and 

 insectivorous habits. In these features Marmosa contrasts strongly with 

 Didelphis, the skull of which in its carnivorous adaptations, resembles that 

 of the Creodonts. In Marmosa both the face and mid- cranial region are 

 short, in Didelphis they are both elongate. In Marmosa the glenoid facets 

 of the squamosals on either side appear relatively further forward; in Didel- 

 phis, by the elongation of the parts anterior to them, they are left near the 

 back of the skull, while the basisphenoid for similar reasons appears very 

 short. In Marmosa the lachrymal is proportionately short, the internal 

 proximal processes of the nasals are more pronounced and the visible portion 

 of the proximal end of the nasals is not so wide. In correlation with the 

 shorter mid-cranial region the constriction back of the orbits is much less 

 pronounced (than in Didelphis), and the orbits also are less removed from 

 the temporal fossse. In the occiput, in consequence of the feeble develop- 

 ment of the crests the mastoid exposure is relatively broader than in Didel- 

 phis and appears more in the side view. The paroccipital processes, as in 

 the skulls of small Insectivores, are barely indicated. 



These adaptive divergences do not conceal an agreement in all the funda- 

 mental marsupial and primitive mammalian characters of the external 

 aspect of the skull, as follows : The basisphenoid in both genera is grooved 

 and pierced by the entocarotid artery (see below, p. 223), while immediately 

 in front of this lies the ''transverse canal" (p. 223). The basioccipital is 

 grooved or pierced on its external border, next to the petrosal, by the posterior 

 carotid foramen (p. 223). The optic foramen is not separated from the 

 sphenorbital (for. lac. ant.) /. e., the nerve does not pierce the orbitosphenoid ; 

 the foramen rotundvm forms a tubular opening immediately Ijehind the 

 sphenorbital. The sphenopalatine (internal orbital) foramen, the stijlo- 

 mastoid, postmastoid and other foramina described below (p. 224) all exliibit 



