Mollusca of the Arabian Sea, &c. 97 



Mactrinula tryphera *, sp. n. (PI. II. fig. 7.) 



M. testa pertenui, subpellucida, lactea, late trigonali ; urabonihus 

 fere centralibus, prominulis, acutis; margine dorsali antice paullum 

 excavate, delude versus marginem ventralem leniter rotundato ; 

 postice breviter extenso, longitudinaliter carinato ; margiue ven- 

 tral! versus latus posticum sinuate, aliter fere recto ; superficie 

 externa omnino usque ad carinam posticam regulariter concen- 

 trice laminata ; laminis circa sex et vigiuti, intus pagina pellucida, 

 laminas externas exbibente ; dentibus cardinalibus et lateralibus 

 parvis, contiguis. 



Alt. 14, lat. 22, diam. 7 mm. 



Hah. Hinderabi Island, at 35 fathoms, mud. 



Shell very thin, mi Iky- vitreous, subi3elhicid, broadly tri- 

 gonal, anteriorly slightly gaping ; the umboiies are almost 

 central, inclining forwards, acute, and prominent ; the anterior 

 portion of the dorsal margin is excavate and at length gently 

 rounding off towards the ventral margin. Posteriorly the 

 margin (dorsal) is shortly extended, longitudinally once- 

 carinate ; the ventral margin is mostly straight, but sinuous 

 where joined by the carina ; the external surface of the shell 

 is uniformly and regularly concentrically laminate, with the 

 exception of the posterior space enclosed by the keel, which 

 is smooth ; the laminge are some six-and-twenty in number, 

 inclusive of those on the um bones, which are small and 

 obscure. Within the surface is pellucid, showing through 

 the external laminge ; the teeth, both cardinal and lateral, are 

 small and near each other. Pallial sinus obscure, hardly 

 perceptible. 



This is a very beautiful and delicate little mollusk, having 

 some resemblance to the larger and coarser M. 'plicataria, L. ; 

 but the form is more regularly trigonal, and the umbones more 

 central, with the anterior dorsal margin less prominently 

 excavate and the concentric laminse closer and finer. 



While discussing a member of the Mactracea, I should like 

 to take the opportunity of stating that further examination of 

 a species described by me as Petricola lyra, from Com- 

 mander Shopland's Aden collections f, prompts one to the 

 decision that it is really a Standella akin to S. Solandri, 

 Gray. The Mactrids of this alliance have a strong super- 

 ficial resemblance to Petricolce, and the teeth of this new 

 form were rather distorted and compressed, thus causing 



* Tpv(}){pos, delicate. 



t Ann. & Mag. Nat. Hist. ser. 7, vol. i, p. 204. 



Ann. & Mag, N. Hist. Ser. 7. Vol. iv. 7 



