58 THE VOYAGE OF H.M.S. CHALLENGER. 



Mactra incarnata, Deshayes. 



Madra incarnata (Deshayes), Reeve, Conch. Icon., vol. viii. fig. 6L 

 TrigoufUa incarnata, H. and A. Adams, Genera Rec. JIoll., vol. ii. p. 376. 

 Tricjimalla incarnata, Tryon, Amer. Journ. Conch., vol. iii., Appendix, p. 37. 



Habitat. — Station 212, .south of the Philijjjjine Mands, iu 10 fathoms; saud. 



This species was described from specimens said to have been obtained at Swan 

 River. Only six small not haLf-grown valves were collected by the Challenger at the 

 above locality. At this age they appear rather more trigonal than when adult, and 

 exhibit more rosy radiation in the interior of the valves. The species is closely 

 related to Mactra eximia of Deshayes, which, however, is a somewhat higher shell, 

 having a more strongly curved basal margin and somewhat coarser sulci on the lateral 

 areas. 



There are three or four species which are characterised with similar lateral grooving, 

 and border rather closely on the present form. Among these may be mentioned 

 Mactra semistriata, Deshayes, Mactra eximia (already referred to), Mactra antiquata, 

 Spengler, and Mactra mera, Deshayes, the two last being unquestionably one and 

 the same shell. The distinction between those and other closely allied forms of 

 Mactra may be very trifling, still they appear fairly constant, each locality producing 

 its variety. 



Mactra isaheUeana, d'Orbigny. 



Mactra isabdleana, d'Orbigny, Voy. Ainerique M^rid., vol. v. p. 509 ; Atlas, vol. ix. pi. Ixxvii. 



figs. 25, 26. 

 Mulinea isabelliana (d'Orbigny), Tryon, Amer. Journ. Conch., vol. iii.. Appendix, p. 31. 



Habitat. — Station 321, off Monte Video, in 13 fathoms; mud. 



D'Orbigny says that this species is found at all points near the mouth of the Eiver 

 Plate at Maldonado, at Monte Video, and on the oj)posite coast at Cape San Antonio. 



It is a moderately solid shell, a little inequilateral, somewhat trigonal, and generally 

 more acute posteriorly than in front. It is of a whitish-brown colour, and more or less 

 covered with a yellowish-olive epidermis. The surface of the valves is sculptured by 

 concentric strise of gro'ni:!!, and generally exhibits, here and there, stronger grooves and 

 elevations, which may indicate periods of arrested increase. The jjaUial sinus is subject 

 to some variation, in some instances being deeper than in others, but it never 

 apparently extends nearly half-way across the valves. The external ligament is minute, 

 situated above the internal cartilage, and is separated from it by a narrow shelly 

 partition. 



