268 172 



Long. 85 mm., alt. 5 mm., crass. 45 mm. 



Between Koh Mesan and Cape Liant, 9 fathoms (3). 



The present species, which somewhat resembles the West Indian species Corb. 

 Kjaeriana, C. B. Adams,' is very characteristic and can easily be distinguished by 

 its form and beautiful sculpture. 



Corbula subquadrata, Melvill. 



Corbula subquadrata, Melvill, Proc. Zool. Soc. London, 1906, p. 843, pi. 56, fig. 7. 



Koh Chuen, 30 fathoms (1). Coast of Lem Ngob, low tide, stones (9). Koh 

 Chang, low tide ('/a). North of Koh Kut, 10 fathoms, mud ('/a). 



Long. 25— 6 mm. (long. 5 mm., alt. 35 mm., crass. 25 mm.). 



Distribution: — Bombay, Persian Gulf, Gulf of Oman. 



This species varies greatly both in regard to outline and convexity, several of 

 the specimens from the Gulf of Siam agree exactly with Melvill's figure, while 

 others differ in outline and are oblong (long. 6 mm., alt. 4 mm., crass. 3'5 mm.); 

 but the peculiar hanging rostrum and the strong sculpture are found in all the 

 individuals. Melvill, in his diagnosis, does not mention that the concentric folds 

 upon the whole surface and also the interstices between them, seen under magnifica- 

 tion, are transversely closely striped; this ornamentation is very characteristic, and 

 occurs in all my specimens. In full-grown specimens there are usually more than 

 "about 12 concentric ribs" (as stated by Melvill), and although in the umbonal 

 region their prominence is extremely slight, yet often at least fourteen fully deve- 

 loped folds may be counted. 



Corbula arcaeformis n. sp. 



(PI. V, Figs. 25—26). 

 The shell is oblong, very convex, clear-white, thin and dully pellucid; it is 

 twice as long as it is high, rounded at the anterior end and drawn out into a point 

 at the posterior end. From the beak a keel extends to the point of the posterior 

 end; the space thereby formed between the dorsal side and the keel, is divided 

 into two halves by a groove which originates at the beak. The middle portion 

 of the shell, especially towards the ventral side, is depressed, and this makes the 

 ventral margin concave. The surface is covered with fine, irregular lines of growth, 

 which towards the ventral side are coarser; these are most pronounced upon the 

 space mentioned above limited by the keel. Under magnification may be seen in 

 the shell-substance a great many fine, white lines, which radiate from the umbones 

 down towards the ventral margin; in quite young individuals and in the umbonal 

 region of well-preserved, older specimens, these lines may be seen as small, fine, 

 irregular, slightly raised stripes. The beaks are situated in the front third of the 



' Dall, Report on the Mollusca coll. by the Steamer "Blake," I, 1886, p. 316, pi. I, fig. 6. The 

 American couchologists erroneously name this species C. Kjoeriana. 



