ANT) AVES OK NORTH AMERICA. 



213 



The exoccipital is distinct, and boars a very small segment of the occipital condyle. 



The opisthotic stands obliquely upwards and forwards, and furnishes a glenoid cavity 

 for the articulation of the quadratum. It has a process directed upwards and forwards, 

 which occupies a concavity on the inner lace of the squamosal, which has the same 

 direction. 



The squamosal is flat below, and proximally presents a longitudinal external angle; 

 medially it is more cylindric, and receives the posterior process of the post-frontal. 



The basioccipital presents a strong transverse condyle. It is a massive bone, and pre- 

 sents infero-laterally two powerful processes which diverge posteriorly, and present broad, 

 rugose, ovate faces of insertion. There is an obtuse keel on the middle line below, which 

 bifurcates posteriorly to each of the lateral processes. The distal portions of these processes 

 are overlapped by corresponding cup-like processes of the basi-sphenoid. 



The basi-sphenoid is distinct from the basioccipital, and underlaps the latter almost to 

 its middle. It is longer than broad, and sends two processes latero-anteriorly to support 

 the pterygoids. These are not so long as in most Lacertilians. Latero-superiorly it pre- 

 sents a broad surface, on each side of the brain case, for support of the upper side walls. 

 Postero-externally, the prootic rests. It thins out anteriorly, and has overlapped the ali- 

 sphenoid. The suture for this bone widens anteriorly; inwardly it is elevated into a low 

 crest of the sphenoid. 



The presphenoid appears to have been distinct; its base; was small; it is lost. 



The floor of the cranial cavity indicates that the medulla oblongata possessed the 

 downward flexure characteristic of Reptiles, but it does not take; place till the middle 

 of the length of the basioccipital is reached. The posterior margin of the sphenoid is 

 marked by a, deep pit; its median floor is a, transverse elevation; it there descends again 

 and terminates in a deep longitudinal groove. 



The roof of the brain-case is marked on the parietal bone by two obtuse; divergent 

 ridges, which leave its posterior margin, and embrace the fontanelle. The grooves for the 

 olfactory pedicels are narrow and well separated, but they unite and are entirely enclosed 

 by inferior processes of the frontal bone, as in serpents and Varani. These ridges separ- 

 ate, and leave the bulbi exposed below. Behind and between the nares the median ridge 

 again appears separating two strong grooves. 



The vomer is divided, and is composed of two slender compressed bones in contact; 

 they arc broken off posteriorly. 



The o. o. pdlatina terminate Ulteriorly opposite the posterior margin of the bony nos- 

 trils; they are widely separated on the middle line, and closely united posteriorly with 

 the pterygoids. The suture connecting the two runs outwards and backwards. 



AMIMMOA. PHILO. SOO. — VOL XTV. 54 



