REPORT ON THE LAMELLIBRANCHIATA. 59 



Mactra achatina, Chemnitz. 



Madra achatina, Chemnitz, Conch. -Cab., vol. xi. p. 218, pi. cc. figs. 1957, 1958. 

 Mactra achatina, Reeve, Conch. Icon., vol. viii. pi. xii. fig. 51. 

 Mactra maculosa, Lamarck, Anim. Sans Vert., ed. 2, vol. vi. p. 100. 

 Mactra maculosa, Hanley, Cat. Reo. Shells, pp. 29 and 340, pi. xi. fig. 27. 

 Mactra ornata. Gray, Ann. and Mag. Nat. Hist., 1837, vol. i. p. 371. 

 Mactra ornata, Eeeve, loc. cit., pi. xiii. fig. 58. 



Habitat. — Admiralty Islands, in 16 to 25 fathoms (Challenger) ; Philippine Islands 

 (Reeve), Ceylon, and Nicobar Islands (Chemnitz) ; var. ornata, China (Gray) ; Ceylon 

 (E. W. H. Holdsworth in Brit. Mus.). 



Among the synonymy of this species given by Reeve and copied by Conrad (Amer. 

 Journ. Conch., vol. iii., Appendix, p. 35) I find mentioned Mactra maculata as of Hanley. 

 The species referred to by the latter author is not the Mactra achatina, but the true 

 Mactra macnlata of Chemnitz, which is a very different shell. It is figured by Reeve, 

 Conch. Icon., fig. 56, and again as Mactra reevei, Deshayes, on pi. xvi. fig. 85. 



The Mactra ornata of Gray I regard as the typical form of this species, and the shell 

 figured by Reeve (fig. 51) as a variety. Specimens in the British Museum from Ceylon, 

 one of the localities mentioned by Chemnitz, agree better as regai'ds colour and form 

 wdth the figure and description in the " Conchylien-Cabinet " than those from the 

 Philippine Islands and Admiralty Islands. 



Mactra incerta, n. sp. (PI. V. figs. 7— 7c). 



Testa mediocriter convexa, ovato-subtrigona, paulo inaequilateralis, tenuiuscula, nitida, 

 levigata, utrinque umbones versus radiatim sulcata, albida, radiis latis interruptis fuscis 

 variegata. Margo dorsi anticus elongatus, oblique leviter couvexus, posticus A^alde 

 declivis, parum arcuatus ; margo inferior late curvatus. Umbones acuti, ad apicem 

 fuscescentes, paulo post medium collocati. Cardo albus, dentibus lateralibus gracilibus 

 munitus. Pagina interna nitida, coloribus externis tincta. Sinus pallii mediocris, 

 rotundata. 



This little species has the anterior side rather longer than the posterior, the former 

 being narrowed towards the end, where it is sharply rounded, and the latter broader, 

 obliquely suljtruncate, terminating in a rounded angle at the base. It is a thinnish shell, 

 rather glossy, with very little sculpture upon the greater portion of the surface, but 

 rather strongly grooved laterally on both sides of the beaks. The ground-colour is 

 whitish, copiously stained with brown in the form of broad interrupted rays which are 

 faintly obscured by an excessively thin epidermal film. The front dorsal margin is a 

 little convex and descends obliquely, the posterior also being slightly excurved, but much 



