_Diagnosis._--Late Pennsylvanian fishes of small size, having the following combination of characters: on basisphenoid, knoblike antotic processes connected by a low ridge to basipterygoid processes; entire ventral surface of parasphenoid toothed; anterior margin of parasphenoid notched and no evidence of hypophyseal opening. Dermal bones of skull smooth or with low, rounded tubercles and striae; fronto-ethmoid shield incompletely known but having one pair of large rectangular frontals with posterolaterally slanting anterior margins; intertemporals large, the lateral margins curving laterally; postorbital triangular, apex downward; subopercular somewhat triangular; squamosal carrying sensory canal that curves down posteriorly and extends onto a ventral projection; opercular generally triangular; supratemporals elongate, curving to fit lateral margin of intertemporals; circumorbital plates lightly ossified. Palatoquadrate complex consisting of endopterygoid and ectopterygoid (both toothed on medial surface), quadrate, and metapterygoid, the latter smooth and having widened border for articulation on anterodorsal margin. Pectoral girdle consisting of cleithrum and clavicle (supracleithrum not seen); small projection on medial surface of posterior portion of cleithrum; horizontal medial process on clavicle. Pelvic plate bearing three anteriorly diverging apophyses, and one denticulate ventromedian process for articulation to opposite plate. Lepidotrichia jointed distally, but not tuberculated. Scales oval, having posteriorly converging ridges on posterior exposed parts. The name refers to the most distinctive character of the genus, the connected antotic and basipterygoid processes on the basisphenoid, and is derived from Greek, _synaptos_--joined, _tylos_ (masc.)--knob, projection. _Synaptotylus_ is excluded from the advanced suborder Coelacanthoidei by the retention of basipterygoid processes on the basisphenoid. _Synaptotylus_ differs from _Rhabdoderma_ in several characters of the basisphenoid, the most important being: knoblike antotic processes (those of _Rhabdoderma_ are wider, more flattened and more dorsal in position); small, lateral basipterygoid processes (in _Rhabdoderma_ these are larger and farther ventral in position). ~Synaptotylus newelli~ (Hibbard) _Coelacanthus newelli_ Hibbard, 1933, Univ. Kansas Sci. Bull., 21:280, pl. 27, figs. 2, 3. _Coelacanthus arcuatus_ Hibbard, 1933, Univ. Kansas Sci. Bull., 21:282, pl. 26, fig. 8; pl. 27, fig. 1. _Rhabdoderma elegans_ Moy-Thomas, 1937 (in part), Proc. Zool. Soc. London, 107(ser. B, pt. 3):399. _Type._--K. U. no. 786F. _Diagnosis._--Same as for the genus. _Horizon._--Rock Lake shale member, Stanton formation, Lansing group, Missouri series, Upper Pennsylvanian.