1869.] 97 [Coues. 



-sphenoid) bounded anteriorly by the alisphenoid, mesially by the 

 bases just mentioned, and posteriorly by the basi- and ex-occipital; 

 it would be a large roundish vacuity were it not for the projection 

 into it from the outside of the large petrosal and inflated tympanic 

 bulla; the fact that these do not reach the occipital base is what 

 causes the merging into one of the posterior and middle fissurs basis 

 cranii. The second opening is a very large one, through which the 

 tip of a finger may be passed, leading into the median optic fossa 

 (conjoined orbital cavities), of an irregular, somewhat cordate shape, 

 circumscribed behind by the alisphenoids, and before by what appears 

 to be an orbito-sphenoid plate (see below) . This gives passage to 

 the optic nerves, as has been said, as well as to the second division 

 of the fifth. The third opening is the irregular foramen in the cu- 

 riously malformed frontal bone, giving exit to the olfactories, and 

 standing in place of a " cribriform plate of ethmoid." These three 

 fissures (the posterior one lateral and divided in two, the anterior 

 two median and single) indicate, in a general way, the places of 

 division of the skull into its four neural arches. 



The zygoma, composed as usual of squamosal and malar, preserves 

 posteriorly its ordinary condition ; anteriorly, however, it curves 

 inward as it passes forward, and meets its fellow on the median line, 

 over the middle of the superior maxillary. A small bone, extending 

 transversely across the median line, serves as a bridge across the 

 anterior extremities of the malars, binding them together in front, 

 and completing a perfect half circle or horse-shoe shaped bony bar, 

 that horizontally surrounds the sides and front of the skull from one 

 glenoid fossa to the other. This bone is formed of the two lachrymal 

 bones fused. The upper jaw hangs below and projects forward from 

 this half-ring, connected behind with the base of the skull by the 

 palatals and vomer. Its superficial resemblance to a lower jaw is 

 very striking. 



The occipital segment offers nothing specially noteworthy. The 

 coalesced ex- and par-occipitals are distinct from basi- and super- 

 occipitals ; all four have nearly if not quite their proper shape and 

 position. 



The basisphenoid is distinct from the basioccipital, but not from 

 the presphenoid; with which latter the next centrum is also fused in 

 great measure, and in fact is less demonstrable as such than usual, in 

 consequence of the extensive blending of the palatals. A vomer is 

 scarcely indicated except by a median longitudinal ridge upon the 



PROCEEDINGS B. S. 2J. H— VOL. XIII. 7 OCTOBER, 1869. 



