Three new genera of eutetrarhynchid trypanorhynch cestodes are described from Mobula spp. (Mobulidae) from the Gulf of California, Mexico. Fellicocestus mobulae gen. et sp. n. from the gall bladder of Mobula japonica (Müller et Henle) is distinguished by elongate bothria, a pars bothrialis equal in length to the pars vaginalis, masses of gland cells in the pars vaginalis and an heteromorphous armature in which hook rows arise from a central file of hooks on the bothrial surface of the tentacle and terminate in a central file on the antibothrial surface. Species of Mobulocestus gen. n. occur in the nephridial system and cloaca of rays and are characterized by two bothria, an heteroacanthous armature with hook rows beginning on the bothrial surface and terminating on the antibothrial surface, and by hooks at the beginnings of rows with an apical cavity. M. nephritidis sp. n. and M. lepidoscolex sp. n., both from the nephridial system of Mobula thurstoni (Lloyd) are differentiated by testis number and by the presence of scale-like microtriches on the tegument of the scolex of M. lepidoscolex. M. mollis sp. n., from the cloaca of Mobula thurstoni is distinguished by testis number (97-111 in M. lepidoscolex, 20-22 in M. nephriticus and 48-70 in M. mollis). Hemionchos gen. n. from the spiral valve of Mobula spp. has two bothria, an heteroacanthous armature, hook rows arising on the bothrial surface and terminating on the antibothrial surface and hooks at the beginning of rows with an apical cavity. It differs from Mobulocestus in having a distinctive basal armature and both hook files 1 and 1' on the bothrial surface, but has an additional, small, satellite hook adjacent to each hook 1'. H. striatus sp. n. from the spiral valve of Mobula thurstoni and M. japonica is differentiated by having a basal armature of closely packed arrays of small, uncinate hooks. H. mobulae sp. n. from the spiral valve of Mobula japonica and M. munkiana Notarbartolo di Sciari, differs in testis number and in having large, flattened hooks in the basal armature. H. maior sp. n., from the spiral valve of M. japonica, is larger, differing in both the number of testes and in the basal armature.
By a sign pattern (matrix) we mean an array whose entries are from the set $\lbrace +,-,0\rbrace $. The sign patterns $A$ for which every real matrix with sign pattern $A$ has the property that its inverse has sign pattern $A^T$ are characterized. Sign patterns $A$ for which some real matrix with sign pattern $A$ has that property are investigated. Some fundamental results as well as constructions concerning such sign pattern matrices are provided. The relation between these sign patterns and the sign patterns of orthogonal matrices is examined.