The fragmentary holotype and only known material of Platecarpus tympaniticus Cope, 1869, is redescribed and compared to other closely related plioplatecarpine mosasaurs. Re-characterization of the holotype reveals the following similarities to Platecarpus ictericus: exoccipital-basioccipital suture transversely oriented in condylar view; basal tubera not inflated and widely separated along ventral midline; cervical intervertebral joint surface strongly curved; seventh cervical vertebra lacking articulating intercentrum (i.e., hypapophyseal peduncle). In addition, the relatively complete right quadrate of the holotype of P. tympaniticus is virtually indistinguishable from those of P. ictericus. Quadrate characters, when combined with the four previously mentioned characters, uniquely diagnose both P. tympaniticus and P. ictericus among known mosasaur taxa. Because P. tympaniticus was established before P. ictericus, the former becomes the specific senior synonym of the latter, and is also the generic type of Platecarpus.