452 O. C. Marsh—Structure of Mosasauroid Reptiles. 
ward, downward and backward. The ilium, which is usually 
the longest of the three pelvic bones, has, in Hdestosaurus, a 
nearly round shaft, slightly sigmoid, and tapering gradually to 
its distal end. This is truncated nearly at mght angles to 
the axis of the shaft, and covered with an articular face, 
There was evidently a cartilaginous union with a vertebra of 
the sacral region, although there was no true sacrum in any of 
the group. This vertebra was probably the last without chey- 
rons, as In a specimen of Lestosaurus, with the pelvic arch com- 
plete, the right ilium was found by the writer adhering to the 
right diapophysis of this particular vertebra. 
The inferior end of the ilium is in Hdestosaurus much di- 
lated, with a sub-rhombic transverse outline, and three terminal 
facets, the largest of which forms part of the acetabulum, The 
P. and t t 
oblique. The bone called “ilium” by Prof. Cope, in his de 
scription of Liodon dyspelor,* is evidently the ischiwm, and we 
versa, as is clearly shown by several very perfect pelvic arches 
in the Yale Museum, some of which were found not only with 
the elements in position, but, what is much more conclusivé 
actually united. Moreover, a careful comparison of these bones 
with the corresponding parts of the pelvic arch in recent lizards, 
as well as in the extinct swimming Saurians, strongly confirms 
the interpretation here given. 
The ischium in Edestosawrus (Plates XI-x11) has the acetabu- 
lar end expanded and thickened, and just below, on the poste 
rior margin, the tuberositas ischii is well develo The sh 
is constricted in the middle, and the distal end has a subtrial: 
gular articular face for union with its fellow, although Prof. 
Cope, in the article above quoted, states that “the pubes are 
below.” In Lestosaurus (Plate X11 
tig. 2, b) the ischium is a broad flat bone with a posterior Pry 
jection on its proximal third. The distal end is expanded, with 
the anterior angle especially produced. This extremity we 
articular face along its fore-and-aft extent, showing clearly ¢ 
it was united below to the opposite ischium. In the genus 
Rhinosaurus, the ischia were also flattened, and the poster? 
process well developed. Their distal ends were moderately 
ss ao and closely united on the median line. ae 
_ The pubis is longer than the ischium in all the genera 12¥® 
tigated. In Hudestosaurus, its extremities are about the same 
* Proceedings American Philosophical Society, Dec., 1871. 
