THE GIRDLES AND HIND LIMB OF HOLOSAURUS 
ABRUPTUS MARSH 
Sk. CAPPS Hii. 
University of Chicago 
The specimen described below, in the Paleontological Collection of 
the University of Chicago, was collected during the summer of 1903 by 
a University of Chicago paleontological expedition. The complete 
skeleton was discovered by Dr. E. B. Branson and collected by him- 
self and Professor Williston. ‘Thanks are due to Dr. Williston for 
permission to study this excellent specimen. 
The specimen is of peculiar interest in that it is probably the 
most complete one of a mosasaur ever collected, almost every bone 
lying in its original position with relation to the rest of the skeleton. 
Furthermore, it belongs to the hitherto very imperfectly known form 
of Holosaurus abruptus Marsh, and the undisturbed condition of the 
skeleton offers an opportunity for clearing up a number of doubtful 
points. Besides this remarkable specimen, a complete pelvic girdle 
of the same species was found at the same time, with the bones spread 
out in their original relations to one another. From the study of these 
two specimens it was determined: (1) that the ilium lay below the 
third pygal vertebra, and not below the first, as usually figured, and 
that it had a nearly vertical position to the vertebral column, instead 
of having a strong backward slope; (2) that there was a firm sym- 
physis of both the ischia and of the pubes, as shown by the position 
of these bones (Fig. 2); (3) that there was an interclavicle bone 
present (Fig. 1). ‘The rear paddle of the mosasaur, moreover, has so 
far been very imperfectly known. It was figured by Marsh in 1880 
for Platecarpus, and by Osborn much later for Tylosaurus dys pelor, 
but neither specimen was perfect, and both are of species different 
from that described below (Fig. 3). 
The pectoral girdle of this specimen is very perfectly preserved 
(Fig. 1). Lying between the coracoids there was found a thin, 
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