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Show 1*90.] MARINE MOLLUSCA OF ST. HELENA. 287 RISSOINA DECIPIENS. (Plate XXIII. fig. 36.; Testa R. bryeriae simillima, sed anfr. ultimo inferne transversim striato, et apertura antice distincte subcanaliculata differt. Longit. 4\ millim., diam. 1|. This species, unless criticalfy examined, might easily be taken for 1. bryeria. It differs in having spiral striae around the lower part of the body-whorl, and the aperture is produced in front into a decided oblique sinus or channel, giving a longer appearance to the mouth. Ihree specimens of this species were presented to the Museum by E. W . Alexander, Esq., in 1857. RISSOINA BRYERIA (Montagu). Turbo bryerius, Montagu, Test. Brit. vol. ii. p. 313, pi. 15. fig. 8 Rissoina bryeria, Schwartz v. Mohrenstern, Denkschr. k. Akad. Wissensch. Wien, 1861, vol. xix. p. 139, pi. v. fig. 36. This is a common West-Indian species, and is also said by Schwartz von Mohrenstern to occur at the Mauritius. The two specimens from St. Helena are intermediate in size between average examples of this species and R. chesneli, and one of them exhibits a distinct indication of the labral tooth of the latter species (vide Schwartz, I. c. fig. 39). No mention of this denticle is made by Michaud, the author of the species ; but iu the figure given by Schwartz von Mohrenstern, taken from a specimen furnished him by Michaud, it is clearly depicted. This feature and its smaller size alone separate it from R. bryeria, and I am inclined, from an examination of a large series of specimens, to believe that neither of these characters are at all reliable, for a perfect gradation in size and in the development of the tooth is observable. I am therefore of opinion that both forms should be regarded in the light of variations of one and the same species. RISSOINA CONGENITA. (Plate XXIII. fig. 37.) Testa R. bryeriae similis, sed minor, costis tenuioribus, magis obliquis instructa, inter costas transversim striata; labrum minus incrassatum. Longit. 3§ millim., diam. lg. The ribs are sharper and more oblique than in R. bryeria or the variety chesneli. The spiral striation is very fine, and chiefly apparent between the costae: if; however, the shells were in very fresh condition, it would doubtless pass over the ribs also. RISSOINA HELENA. (Plate XXIII. fig. 38.) Testa parva, albida, subpellucida, ovato-lurrita ; anfractus 6, duo supremi laevigati, perconvexi, cceteri mediocriter convexi, sutura profunda sejuncti, costis oblique curvatis 15-16 instructi, undique minute spiraliter striati; apex peculiar is, magnus; apertura obliqua, subpyriformis,longit. totius 3' sub- 20* |
