_Diagnosis._--Sphenethmoid region partly ossified, and consisting of basisphenoid, parasphenoid, and ethmoid ossifications; paired basipterygoid process and paired antotic process on basisphenoid; parasphenoid of normal size, and closely associated with, or fused to, basisphenoid; ethmoids paired in _Rhabdoderma_ (unknown in _Synaptotylus_). _Discussion._--Because of the great differences in endocranial structure between the Devonian and Pennsylvanian coelacanths, they are here placed in new subfamilies. The two proposed subfamilies of the family Diplocercidae are the Diplocercinae and the Rhabdodermatinae. The Diplocercinae include those coelacanths having two large unpaired bones in the endocranium (at present this includes _Diplocercides_ Stensioe, _Nesides_ Stensioe and _Euporosteus_ Jaekel). The subfamily Rhabdodermatinae is composed of coelacanths having reduced endocranial ossification, as described in detail above, and now including _Rhabdoderma_ Reis and _Synaptotylus_ n. g. Members of this subfamily differ from those of the subfamily Diplocercinae in having several paired and unpaired elements in the sphenethmoid region of the endocranium, instead of only one larger ossification. They differ from those of the suborder Coelacanthoidei in the retention of basipterygoid processes. _Synaptotylus_ is more closely related to _Rhabdoderma_ than to the Diplocercines because the anterior portion of the endocranium contains only a basisphenoid, parasphenoid, and probably ethmoids. The sphenethmoid region was certainly not a large, unpaired unit as in the Diplocercines. Probably the posterior part, the otico-occipital region (not known in _Synaptotylus_), was much more nearly like that of _Rhabdoderma_, which consisted of unpaired basioccipital and supraoccipital, and paired prootics, exoccipitals, and anterior and posterior occipital ossifications (Moy-Thomas, 1937: figs. 3, 4). Moy-Thomas (1937:389) points out that in _Rhabdoderma_ the occipital region is "considerably more ossified" than in any coelacanths other than the Devonian forms. Berg (1940:390) thought that the Carboniferous coelacanths should be placed in a separate family because they did not have two large, unpaired bones in the endocranium. _Rhabdoderma_ and _Synaptotylus_ represent another stage in evolution of the endocranium in coelacanths, and, if classification is to be based on endocranial structure, then this stage (represented by the two genera) may later be given family rank as Berg suggested. Because _Rhabdoderma_ and _Synaptotylus_ have only part of the sphenethmoid region ossified and because they retain basipterygoid processes, they are considered to be related and are included in the subfamily Rhabdodermatinae. ~Synaptotylus~, new genus _Type species._--_Synaptotylus newelli_ (Hibbard). _Horizon._--Rock Lake shale member, Stanton formation, Lansing group, Missouri series, Upper Pennsylvanian.