84 MEMOIRS OF THE CARNEGIE MUSEUM 
breadth in flaring out to meet the investing bones. As usual it is separated from 
the articular tract of the condyle below by a very shallow groove. In both speci- 
mens (Nos. 603 and 878) inferiorly there is a very prominent transverse dividing 
line, which probably represents the deep vertical suture between the basioccipital 
and basisphenoid. In either instance however the size of the suture has probably 
been exaggerated somewhat by crushing. On the median neural surface there is a 
shallow antero-posterior channel which forms the basal boundary of the foramen 
magnum, and on either side are deep excavations for the sutural articulations with 
the exoccipitals. These elements do not enter into the composition of the oc- 
cipital condyle as in some reptiles. 
Principal measurements are as follows: 
No. 878. Greatest breadth of occipital condyle..................:.ceeee eeeeeeee eres 65 mm. 
te 603. ce oe oe (73 . “cc 
“ 878. & d epth “ce oe ce 
“ 603. ec “et oe “eo “ 
uC fSizteh Gs breadth of basioccipital : 
ce 603. oe “e “cc “ce 
Oo Sateh tf depth sf a 
“ 603. “cc ce oe “ 
uS ie of length se a 
603. ss ss OG ( 
Exoccipitals (ex.occ.). —These elements viewed from the back are seen as two 
small subrectangular bones that rise from the superior surface of the basioccipital 
to support the heavy inverted U-shaped supraoccipital. The interior margins of 
these bones form the walls of the lower third of the foramen magnum. Below the 
articular ends are somewhat enlarged for the strong sutural articulation with the 
basioccipital. The external surfaces are very slightly concave transversely. These 
elements do not show the lateral excavations seen in the exoccipitals of Ichthyo- 
saurus, but apparently there is a foramen at this point as in the former genus 
though this cannot be definitely determined from our specimens. 
MEASUREMENTS. 
No. 878. Greatest length of exoccipital....................ceeceeeeee eee eeeeeceeeceees 34 mm. 
3G Bis}. ‘¢ breadth ss 
Supraoceipital (s.oce.). — Of the four bones forming the boundary of the foramen 
magnum the supraoccipital contributes the greater part. ‘This element as seen 
posteriorly has the form of an inverted U, the stems of which articulate with the 
exoccipitals. The supraoccipital of B. discus differs materially from the correspond- 
ing element in Ichthyosawrus. In the latter genus the supraoccipital is a remarkably 
