WILLISTON: RESTORATION OF KANSAS MOSASAURS. LOQ 
| that of the Z'y/osaurus, though the carpus and tarsus are less well i 
developed than in the former. The hind paddles are only slightly 
smaller than the fore ones, and all are powerful propelling organs, i 
far more so than in any other known genus of the group. Alto- i 
gether, in proportion to its size, Platecarpus was the most powerful 
| and predaceous of the Kansas Mosasaurs. It will be observed that 
the teeth in this form, while not as numerous as in C@dastes, or iF 
| as stout as in 7'y/osaurus, are more effective weapons than in either ; 
of these genera, being more elongated, more curved and more 
° Pointed. The neural spines do not form as close a series as in 
Tylosaurus, indicating greater flexibility. 
In Zylosaurus we have in some respects the most specialized of ie 
| the Mosasaurs. The almost wholly cartilaginous carpus and 
tarsus, the more elongated digits and the greater number of the 
| Phalanges, are characters brought about by aquatic habits. On | 
the other hand, the hind paddle is actually larger than the front, i. 
| and the fifth digit has undergone little or no reduction, characters 
| of a more primitive rank. The paddles are more flexible than in 
| either of the other genera, but they are relatively small and not at 
| all Strong. The skull is more elongated anteriorly and there is no 
| trace of a zygosphene. 
| Dr. Dollo has expressed a doubt of the nature of the vertebra 
Called pygial by Mr. Case and myself in a former publication. 
He believes that some of them at least are true lumbar vertebrae, 
as all were previously thought to be. I feel yet more assured that 14 
they are basal caudal and have so restored the different genera. 
In the living lizard, with the sacral synchondrosis, the ilium is di- ; 
fected forward, throwing the symphysis ischii below the sacrum 
| and leaving the outlet of the pelvis unrestricted. In Varanus there 
are as few as two non-chevron-bearing vertebra back of the sacrum. 
More were not needed. In these marine lizards, on the other 
hand, the shaft of the ilium is directed obliquely forward, bringing 
the symphysis of the ischii below the fourth or fifth of the vertebre 
Succeeding the ligamentous attachment. If these or any of them 
bore chevrons, it will be immediately seen that they would project 
nto the cavity of the pelvis. Not less than six pygal vertebra are 
necessary to leave space for the free exit of the cloaca. The ilium 
Must have been in every case attached to the first non-costiferous 
vertebra. 
In these three species the number of vertebra in the different 
regions may be given as follows: 
| 
} 
| 
) 
| 
