360 Mr. Broderip and Mr. G. B. Sowerby on some 



crassdy olivaced; valvis striatis, striis obliquis lineas incrementl 

 decussantibus ; long. 1|, alt. f, lat. /g, poll.* 

 Hah. in Oceano Arctico. 



One of the largest of the Nuculse, in some respects resembling a fos- 

 sil species fomid in the Crag of Suffolk. It is of an oblong shape, the 

 anterior being longer than the posterior side : it is covered with a strong, 

 shining olivaceous epidermis : the anterior side is somewhat rounded, and 

 the posterior produced into a slightly recurved beak. The surface of 

 each valve is covered with delicate concentric lines of growth, which are 

 crossed on the central part, and towards the posterior side of the valves 

 by stronger oblique striae. When the valves are closed, the upper poste- 

 rior edge forms a sharp elevated ridge. The oblique striae are distinct on 

 the epidermis as well as on the shell. Two or three specimens were ob- 

 tained in Vatcha Bay, Kamtschatka. 



Mactra pallida. 



M. testa ovato-trigond, convexd, antice rotundatd, postice sub-carinatd, 

 albidd, sub-pellucidd, umhonibus distant Ibus ; epidermide tenui, 

 corned; long. 2^\, lat. l-^%, alt. ly^^, poll. 

 Hah. ad littora Oceani Pacifici. 



Approaching in form to M. Stultorum, but differing from that shell 

 especially in the carination of the posterior part of each valve, and in 

 the distance of the umbones. From St. Bias. 



* Mactra subglobosa. 



M. testd subglobosd, antice rotundatd^ postice rotundato-angulatd, albi- 



* To prevent an inaccurate conclusion as to the proportions, the reader is 

 requested to observe this explanation of the terms used in describing the bi- 

 valves. Length — the distance from the anterior to the posterior extremity. 

 Breadth — the space between the external surfaces of the valves when closed. 

 Depth or height — the distance from the basal margin to the highest point of the 

 umbones. The bivalve, thus measured, is supposed to rest on the basal mar- 

 gins of the valves, with the umbones uppermost. The anterior part Which may 

 be, generally, known from its comparative shortness, and the turn of the um- 

 bones towards it, is the place where the mouth or the animal is situated. 



