150 We Jal IOBITS 
difference in the relative position of the external nares in the two 
genera, since the other openings occupy similar positions in both. 
There are also some minor differences in the shapes of the various 
openings of the skull. 
The thanks of the writer are due to Dr. S. W. Williston, whose kind 
personal supervision and interest have made this work possible. Dr. 
Williston has not only given the study his attention during its prose- 
cution and the benefit of his intimate knowledge of reptilian types, 
but he has also criticized the completed paper. The writer’s thanks 
are also due him for permission to make use of the material upon 
which the paper is based. 
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