ISPO.") MARINE MOLLUSCA OF ST. HELENA. 28/ 



RissoiNA DECiPiKNS. (Plate XXIII. fig, 36.; 



Tenia R. bryerise simillitna, sed anfr. ultimo inferne tranncersim 

 sfriato, t;t apertura antice distliicle siibcanaliciilata differt. 



Loiu/if. 4| milliin., dinm. \h. 



This species, unless critically examined, might easily be taken 

 for P. bryeria. It clifters i:i 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. 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. 31.3, pi. 1.5. 

 fig. 8. 



Rissoina bryeria, Schwartz v. Mohreustern, 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 Mohreustern 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 in the tigure given by Schwartz von Mohreustern, 

 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. 1 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 sinulis, sed minor, costis temUorihus, magis 

 obliquis instructa, inter castas transversim striata ; labrum 

 mi/ius incrassatum. 

 LoiKjit. 3§ millim., diaiii. Ig. 



The ribs are sharper and more oblique than in R. bryeria or the 

 variety chesneli. The spiral striation is very tine, and chiefly 

 apparent between the costse ; if, however, the shells were in very 

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



Rissoina helentE. (Plate XXIII. fig. 38.) 



Testa parva. albidii, sabpellucidd, ovato-turrita' ; anfractus G, 

 duo sai>reini heciyati, percoitcexi, cteteri mediocriter convexi, 

 sutura profunda sejuncti, costis oblique curvatis 1.5-16 in- 

 structi, uudique minute spiraliter sfriafi; apex pcculiaris, 

 mutfnus ; apertura obliqua, subpyriformis, louyit. totius \ sub~ 



20* 



