GILMORE: OSTEOLOGY OF BAPTANODON (MARSH) 87 
openings, furthermore, it would appear that nearly if not all of these figures are 
either diagrammatic or drawn from reconstructed material which may explain some 
of the otherwise unaccountable differences, not only between the forms of the two 
genera compared here, but in species of the genus Ichthyosaurus. Differences which 
if present should certainly be considered as of more than specific importance. 
Cope” in 1870 was the first to recognize and correctly determine the opisthotic 
and stapedial bones in Ichthyosaurus. Since then, however, as shown above some 
authorities have given posterior views with the stapes absent. 
MEASUREMENTS. 
No. 878. Greatest length of stapes............002...ceeceeeeeceeneeceeeeeees 70 mm. 
OG tlelsh canebreadthyofyproximaliendsesreeeatesesceee ses eee eeeseee cence se 
OS Sith of fs distalvend ise. vec teresa ae aeons 225i 
Squamosal (sq.).— In the nomenclature of this bone the usual determination is 
followed. Both Baur and Merriam reverse the names of the two bones here desig- 
nated as syuamosal and swpratemporal. The element here called squamosal is the 
mastoid of Owen. 
The squamosal is a large irregular triradiate bone, the upper part of which 
forms the superior posterior angle of the cranium. From a posterior obtuse central 
portion three branches are given off. The internal branch, the most robust part of 
the element extends forward and upward from the angle of the skull to meet the 
lateral posterior margin of the descending parietal. Superiorly the surface is thick- 
ened and rounded and forms the posterior boundary of the supratemporal fossa. 
Seen posteriorly it is a flattened subtriangular plate the base of which laps over the 
upper lateral margin of the opisthotic. The inferior plate superiorly is deeply 
indented by the intrusion of the superior end of the opisthotic, or in reality this 
element is opposed to the squamosal at the junction of the internal and inferior 
parts. The inferior branch as it descends widens into a broad flat plate that com- 
pletely fills all of the space between the opisthotic, stapes and quadrate, its lower 
extremity passing under the upper lateral margin of the stapes. The outer border 
unites with the inner border of the quadrate for the greater part of its length. 
Medially this plate is inclined forward, its inner margin underlapping both the 
opisthotic and stape. ‘The superior outer margin is continued upward along the 
quadrate to meet the supratemporal at the angle of the skull. The anterior branch 
of the squamosal is greatly compressed from side to side. This thin plate is directed 
forward and inward, and forms about one half of the outer boundary of the temporal 
fossa. Although it is produced along the inner side of the postfrontal by an elon- 
Cope, E. D., lit. cit., p. 247. 
