I thank Prof. Theodore H. Eaton, Jr., for suggesting the study here reported on, for his perceptive criticisms regarding it, and for his continued patience throughout my investigation. Financial assistance was furnished by his National Science Foundation Grant (NSF-G8624) for which I am also appreciative. I thank Dr. Rainer Zangerl, Chief Curator of Geology, Chicago Museum of Natural History, for permission to examine the specimens of _Captorhinus_ and _Dimetrodon_ in that institution. I am grateful to Mr. Robert F. Clarke, Assistant Professor of Biology, The Kansas State Teachers College, Emporia, Kansas, for the opportunity to study his specimens of _Captorhinus_ from Richard's Spur, Oklahoma. Special acknowledgment is due Mr. Merton C. Bowman for his able preparation of the illustrations.
The outlines of the skulls of _Captorhinus_ differ considerably from those of the skulls of the primitive captorhinomorph _Protorothyris_. Watson (1954:335, Fig. 9) has shown that in the morphological sequence, _Protorothyris--Romeria--Captorhinus_, there has been flattening and rounding of the skull-roof and loss of the primitive "square-cut" appearance in transverse section. The quadrates in _Captorhinus_ are farther from the midline than in _Protorothyris_, and the adductor chambers in _Captorhinus_ are considerably wider than they were primitively. Additionally, the postorbital region of _Captorhinus_ is relatively longer than that of _Protorothyris_, a specialization that has increased the length of the chambers within.
In contrast with these dimensional changes there has been little shift in the pattern of the dermal bones that roof the adductor chambers. The most conspicuous modification in _Captorhinus_ is the absence of the tabular. This element in _Protorothyris_ was limited to the occiput and rested without sutural attachment upon the squamosal (Watson, 1954:338); later loss of the tabular could have had no effect upon the origins of muscles from inside the skull roof. Changes in pattern that may have modified the origin of the adductors in _Captorhinus_ were correlated with the increase in length of the parietals and the reduction of the supratemporals. Other changes that were related to the departure from the primitive romeriid condition of the adductors included the development of a coronoid process, the flattening of the quadrate-articular joint, and the development of the peculiar dentition of _Captorhinus_.