Hypelasma Paquier 1898

GenusHypelasmaPaquier, 1898 Type species. Hypelasma colloti Paquier, 1898, by monotypy. Remarks. Hypelasma was considered monospecific and diagnosed as very inequivalve by Paquier (1898), with a non-spirogyrate right valve and a much larger left one with a prominent umbo. Dentition was described as rather robust, with the posterior socket 3 b’ occupying a major part of the hinge plate in the left valve. In the right valve, the posterior adductor muscle inserts on a downward extension of the hinge plate, while it inserts on a weak myophore in the left valve. This diagnosis was emended by Gourrat et al. (2003), who included a second species, Hypelasma salevensis (Favre in Joukowsky and Favre 1913), in the genus, to comprise small to medium-sized requieniids with slowly expanding left valves with a prominent, projecting umbo. An initially rather acute but progressively more rounded carina in the left valve separates a smaller flattened anterior flank, from a well-rounded posterior one. The right valve is posteriorly carinate. In the left valve, the posterior adductor muscle inserts on a small but distinct posterior myophoral ledge below the hinge plate; in the right valve it inserts on the downward extended hinge plate. The anterior adductor muscle inserts on the shell wall in both valves. Here, we include a third, new species in Hypelasma, which shares all of these characters apart from the pronounced carinae in either valve.