OSTEOLOGY OF CARNIVOROUS DINOSAURS. 



13 



Basisphenoid (b. s.). — The basisphenoid in Antrodemus has a most unusual 

 ventral development, its lower border extending below the level of the basioccipital 

 processes. This part of the basioccipital consists of two lateral plates of bone, 

 broad antero-posteriorly that diverge from one another in a downward direction. 

 Between these plates on the inner posterior sides the basioccipital processes are 

 received, and the posterior borders unite by suture with the ventral extension of the 

 paraoccipital processes. Thirty millimeters posterior to their anterior margins a 

 transverse bridge connects the two lateral plates and completes the boundaries of a 

 very deep rectangular, wedge-shaped cavity which extends upward to a point above 

 the level of the lower extension of the alisphenoids shown in a lateral view (fig. 6). 

 This cavity probably leads up to 

 the foramen for the median Eus- 

 tachian canal, as in the crocodile 

 and alligator. The structure of 

 this part of the basisphenoid is 

 quite different from that of Tyran- 

 nosaurus which apparently has a 

 longitudinal bridge with a cavity 

 od either side. 



In front of the bridge men- 

 tioned above the lateral plates thin, 

 terminating in a relatively acute 

 edge and on the upper half, meet- 

 ing at the median line. On the 

 lower half they turn gently out- 

 ward away from the center, their 

 lower anterior border developing 

 vertically, elongated, oval basi- 

 pterygoid processes for articulation 

 with the pterygoids (b.,pt., pr., figs. 

 4 and 6) . Superiorly the basisphe- 

 noid unites with the prootics and 

 alisphenoids, the latter sending 

 down long flattened processes that 

 lap along the median part of the 

 plate-like ventral extension. 



ParaspTienoid (p. sp.). — The parasphenoid is missing in this specimen though 

 doubtless it continues out from the anterior upper border of the basisphenoid as in 

 Tyrannosaurus. It is present in the American Museum specimen, as shown in 

 figure 16. 



OrbitospJienoid (o. sp.).— The orbitosphenoids are two curved plate-like bones 

 lying between and in front of the alisphenoids, united by suture on the midline, and 

 forming that part of the brain case which underlies the olfactory lobes. These 

 bones, as Gregory 1 has pointed out, may in part also represent the presphenoids, 

 but for convenience in referring to them are here designated orbitosphenoids. 



lot; 



Fig. 6. — Skull of Antrodemus valens Leidy viewed from in front 

 and below. no. 4737, u.s.n.m. \ nat. size. alsq alisphenoiii; 



6. pt. pr, BASIPTERYGOID PROCESSES OF THE BASISPHENOID; bs, BASIS- 

 PHENOID; ex. oc, exoccipital; /, frontal; n. s, suture for nasal 

 bones; o. sp, orbitosphenoid; p, parietal; p. oc, paraoccipital 

 process; p. of, postfrontal; pro, prootic; sq, squamosal; I, II, 

 foramina for exit of cranial nerves one and two. 



1 Journ. Morph., vol. 24, no. 1, March, 1913, pp. 11-12. 



