1^90.] MARINE MOliLUSCA OF ST. HELENA. 287 



RissoiNA DECiPiENS. (Plate XXIII. fig. 36.; 



Testa R. bryerise sljiiillima, sed anfr. ultimo inferne transversim 

 striuto, tit apertura antice dlstincte subcanaliculafa differt. 



Lonr/it. 4-1 millim., diam. \\. 



This species, unless critically examined, might easily be taken 

 for P. bryeria. It differs i:i having spiral strise 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. Three specimens of this species were presented to the 

 Museum by E. W. Alexander, Esq., in 18.57. 



RissoiNA BRYERIA (Montagu). 



Turbo bryerius, Montagu, Test. Brit. vol. ii. p. .Sl.S, pi. 1.5. 

 tig. 8. 



Rissoina bryeria, Schwartz v. ^lohreustern, Denksclir. 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 vou Mohreustern to occur at the Mauritius. The two 

 specimens from St. Helena are intermediate in size between average 

 examples of this species and R. chesiieli, 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 in 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 exaniiuation 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 tlie 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. bryerise siiiiilis, sed minor, vostis tenuioribus, inagis 

 obliq^uis instructa, inter castas transversim striata; labrum 

 minus incrassatum. 

 Longit. 3§ millim., diam. Ij. 



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 costse ; if, however, the shells were in very 

 fresh condition, it would doubtless pass over the ribs also. 



Rissoina helen^. (Plate XXIII. fig. 38.) 



Testa parvu, albidu, sub^jellucida, ovato-turrita, ; anfractus G, 

 duo su^iremi latvigafi, percoavexi, cteteri mediocriter convexi, 

 sutura profunda sejuucti, costis obU<^iue curoatis I0-I6 /»- 

 structi, imdiciue minute spiraliter striati; apex pevuliaris, 

 magnus ; apertura oblifiua, subpyriformis, longit. totius \ sub~ 



20* 



