MORPHOLOGICAL REVISION 



71 



dividing partition there is a deep pit, probably the openings of the Eustachian 

 canals. In the Pelycosauria the canals open into a single median pit. 



Posterior to their articulation with the basipterygoid processes of the basi- 

 occipital the pterygoids pass backward as vertical plates to join the quadrate, but 

 the vertical plates are curved so far as to follow approximately the outer wall of the 

 skull, leaving no great space between them. Anteriorly, the pterygoids nearly, 

 but not quite, meet in the median line; passing forward they articulate with the 

 inner edges of plates attached to the maxillaries. and continuing forward pass above 

 these plates so that they are underlaid by them. These plates occupy the position 

 of palatines and probably are such, but their position underlying the pterygoids 

 is very peculiar. In other specimens, No. 1078 University of Chicago, an imperfect 

 ectopterygoid can be traced, but none can be made out in this specimen. It is a 

 very small element lying between the outer end of the pterygoid and the maxillary. 



Fig. 22. — Skull ol D. phascolinus. X i No. 4839 Am. Mus. 

 A, transverse section; B, longitudinal section of same, a/, alisphenoid; als.r, alisphenoid of right side; 

 bsp, basisphenoid; bo, basioccipital; eth t ethmoid; pt, pterygoid; pv.r y prevomer of right side; 

 mx.r, maxillary of right side; 0, orbit; v, opening for escape of cranial nerves. 



Between the palatines (maxillary plates of Cope) there is left quite a space, in 

 which lie the prevomers. These are vertical plates of considerable height, but 

 very narrow on the palatal surface. Towards the posterior ends the upper edges 

 of the plates flare outward to join the palatines or pterygoids, but at the anterior 

 end the plates are vertical and parallel; the upper edge touches the lower edge of a 

 median vertical plate described below. There are traces of a few large conical 

 teeth on the prevomers, but they are sparse and irregularly arranged. 



Longitudinal section of the skull: The brain case shows the sharp downward 

 bend posterior to the epiphysis described by Cope in his account of the cast of the 

 brain cavity. Anterior to the optic region there is a very large foramen which gave 

 exit to the seventh nerve; it is likely that other nerves escaped through the same 

 opening. Anterior to this foramen the walls of the brain case are formed by solid 

 elements, the all sphenoids, which are attached to the under side of the roof above 

 and to each other and the basisphenoid below. They may be traced as far forward 



