THE SKULL OF LABIDOSAURUS. 
BY 
S. W. WILLISTON, 
University of Chicago. 
WITH 3 PLATES. 
There are not a few problems in the morphology of the reptilian 
skull which yet await satisfactory solutions. Both paleontology 
and embryology have failed hitherto to determine indisputably the 
homologies of the bones of the temporal region, scarcely any two 
authors being in accord in the use of the terms applied to some of 
them. Not only is there yet doubt as to the real structure of certain 
parts of the skull, but the relationships of many groups, dependent 
as they are so intimately upon cranial homologies, are in dispute. 
It is evident that the final solution of these problems, problems 
which resolve themselves at last into questions of relationships, must 
come from paleontology, and especially from the study of the more 
primitive and generalized land vertebrates. 
Of the paleozoic reptiles the structure of the skull is perhaps best 
known in the Pelycosauria, thanks chiefly to the studies of Case in 
recent years. A recent review of the material in this group of rep- 
tiles preserved in the University of Chicago museum, among the best 
now known, that which served Case for the most part in his studies, 
has convinced me of the general trustworthiness of his results, though, 
as will be seen, I am not fully satisfied as to the legitimacy of some 
of his conclusions. That the Pelycosauria are unrelated genetically 
with the archosaurian reptiles | am now convinced; that they did 
arise from that branch of reptiles usually called Cotylosauria in the 
narrow sense, I am also convinced. The resemblances of the skeletal 
characters between the Cotylosauria and the Pelycosauria have long 
been evident; the modifications of the pectoral and pelvic girdles in 
THE AMERICAN JOURNAL OF ANATOMY.—VOL. 10, No. 1, JAN., 1910. 
