ovement of a skeletal member.

Both direct anatomical evidence and inferred functional relations were
used to satisfy the purposes of the study here reported on. The
following account reports the results of my efforts to: 1, reconstruct
the adductor muscles of the mandible in _Captorhinus_ and _Dimetrodon_;
2, reconstruct the external adductors of the mandible in the cynodont
_Thrinaxodon_; and 3, learn the causes of the appearance and continued
expansion of the temporal fenestrae among the reptilian ancestors of
mammals.

The osteology of these three genera is comparatively well-known.
Although each of the genera is somewhat specialized, none seems to have
departed radically from its relatives that comprised the line leading
to mammals.

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.

Captorhinus

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 con