GILMORE: OSTEOLOGY OF BAPTANODON (MARSH) 89 
massive bone which only enters slightly into the formation of the foramen magnum, 
hardly more than arching over the superior boundary. 
The supraoccipital and exoccipitals do not maintain a vertical position as in 
most reptiles, but are inclined forward at a considerable angle. The superior and 
lateral surfaces of the former being wedged in between and under the overlying 
parietals, not grown fast with them but as in Jchthyosawrus only held there by liga- 
ments or cartilage. The posterior face from the edge of the foramen to where it 
passes under the parietals presents a slightly rounded surface. On the superior 
median surface this element is slightly enlarged, see Pl. XL., fig. 1 and 2, resembling 
somewhat the complete symphysis of a suture, which suggests the idea that these 
elements might have been paired at one time. The articular ends are triangular in 
form with the central portion slightly concave. 
It may be of interest to note that the part this element takes in the formation 
of the foramen magnum, 7. ¢., the triangular form of the articular face for the ex- 
occipital and the suggestion of haying been paired at one time, it resembles the 
Plesiosaur (Dolichorhynchops osborni)" more than any other form of reptile that has 
come under the observation of the writer. 
MEASUREMENTS. 
No. 603. Greatest height of supraoccipital 6 70 mm. 
ce 878. “cc “ce “ce 65 “cc 
OC Bhrfeh ‘¢ width £s BREE ereirionOned sneer cerecanepaen 26)iias 
Opisthotic” (op.o.).—The opisthotics are two subcylindrical bones that extend 
outward and upward from the lateral superior surfaces of the basioccipital, broadest 
where they meet the basioccipital, exoccipital and stapes, constricted medially, 
again expanding into a rounded end which abuts against and between that portion 
of the squamosal where it divides into a superior and inferior branch. The squa- 
mosal really encloses the upper half of these elements. The lower articular end 
has three distinct articular faces which unite with the exoccipital, basioccipital and 
stape respectively. It will be observed in Pl. XL, fig. 1, that the right opisthotic 
is somewhat removed from what I have considered as the normal position shown 
by the left element. The relations and shape of these elements differ considerably 
from the opisthotics in Ichthyosawrus. Here we find the upper or superior end 
articulating with the squamosal and not free as shown by most of the illustrations 
of this view of the skull of Ichthyosawrus. 
19 Williston, S. W., ‘‘ North American Plesiosaurs,’’ Part I., Field Columbian Museum, Publication 73, Geol. 
Series, Vol. II., No. 1, fig. 3, p..27. 
20Tt has been shown by Dr. S. W. Williston that this element if a single bone should be called the paraoccipital 
which is the older name given to this bone by Owen in 1838. But until better understood I use the generally accepted 
term opisthotic, Jit. cit., p. 25. 
