Emendation of the generic diagnosis of Tylocephalum (Cestoda : Lecanicephalidea : Tetragonocephalidae), and description of Tylocephalum brooksi n. sp

A new species of Tylocephalum is described from the spiral intestine of the cownose ray Rhinoptera bonasus taken off Venezuela. Tylocephalum brooksi n . sp. is the third species described parasitizing this host species, along with T. pingue and T. bonasum. T. brooksi is similar to congeners in the po ssession of a scolex with a fungiform pars apicalis, craspedote segmentatio n, circummedullary vitellarium, external seminal vesicle, bilobed ovary, an d medioventral uterus. However, this species differs from all previously de scribed species of Tylocephalum in having a collar around the base of the p ars apicalis densely covered by large digitiform processes (microtriches?), and an internal seminal vesicle. The internal seminal vesicle is a feature currently used to differentiate Tetragonocephalum from Tylocephalum. Its p resence in T. brooksi indicates that it should be considered a specific ins tead of a generic character. An emended diagnosis of Tylocephalum is propos ed excluding this feature, along with distribution of the testes in the pre ovarian field and circummedullary distribution of vitelline follicles. Spec ies of Tylocephalum and Tetragonocephalum are reviewed on the basis of orig inal descriptions, and 2 distinct morphological groups of species defined. Species of the Tetragonocephalum group possess a combination of an acrasped ote strobila, segments much longer than wide, testes limited to the region anterior to the cirrus sac, vitellarium of lateral bands, conspicuously enl arged genital atrium and pore, cirrus sac dorsal to genital atrium, and bis accate uterus. Species of the Tylocephalum group differ from Tetragonocepha lum, in possession of a craspedote strobila, segments more quadrangular, te stes distributed throughout the preovarian field, inconspicuous genital atr ium, uniformly saccate uterus, cirrus sac lateral to genital atrium, and pr obable circummedullary vitellaria in all species.