87 183 



The sirand off the mangrove at the station on Koh Chang (1). OIT Koh Kut, 

 5 fathoms (2). Coast of Koh Kong ((5). 



Long. 3 — 7 mm. 



Distribution: — Ceylon, Aden (Shopland). — South of New Guinea, Torres 

 Strait. 



Montacuta (Tellimya) triangularis, A. Ad. 



l'ulliinci tiidniiuhiris, A. Aimms, I'roc. Zool. Soc. London, XXIV^ 1856, p. 47, No. 5. 



— — — Eixi. Smith, Annals and Mag. of Nat. Hist., vol. VIII, 1891, p. 231, No. 8. 



Mactra nucleus, Reeve, Conchol. icon., VIII, Mactra, Sp. 102 (non M. nucleus, Conuad, .Journ. Acad. 

 Nat. Sc. Philadelphia, VI, part 2, 183U, p. 258, No. 3). 



Koh Kahdat ('/s). S. of Koh Kahdat, 8 10 fathoms, mud ('/i). N. of Koh 

 Kut, 10 fathoms, mud Qi-i). 

 Long. 5 — 7 mm. 



Distribution: — Manilla (A.Adams). 

 Kellya siibsinuata, Lischke, resembles the present species very much in form. 



Montacuta (Tellimya) variabilis n. sp. 



(PI. Ill, Figs. 30—32). 



This species is triangular in form, dully white, semitransparent, rather tiiin 

 and flat. The umbones, which are situated in the front half of the length of the 

 shell, sometimes quite anteriorly, sometimes nearer to the middle of the shell, 

 are pointed and terminate in a smooth, round tubercle; the embrj'onic shell is 

 very distinct. From the umbones the upper margin slopes abruptly downwards 

 on both sides. The ventral margin is sometimes convex, sometimes straight, and 

 sometimes somewhat concave. The concentric lines of growth make the surface 

 rough and are sometimes coarse. The interior of the valves is glossy, and there 

 are, in the left valve, two divergent, flat, pointed, triangular, highly prominent car- 

 dinal teeth, separated from the upper margin by a groove; between these teeth 

 there is a triangular space in wliich are ])]aced the cartilage and the ossicle, these 

 fit into a corresponding triangular space in the hinge plate of the right valve. 

 Directly upon the upper margin of the valve and close to the hinge plate there is 

 in the right valve, on each side, a slightly developed lateral tooth, of which the 

 anterior is the larger; these teeth fit into the groove between the teeth and the upper 

 margin of the left valve. 



Long. 4 mm., alt. 3 mm., crass. 2 mm. 



The Gulf at the south end of Koh Chang (6). 



As may be seen from the above diagnosis, this species varies highly in form, 

 a fact which is undoubtedly due to its habitat and mode of living (commensalism?); 

 the characteristic triangular form is common to all the individuals. I have figured 

 on PI. Ill, Figs. 30-31 the two most divergent forms, which limit the variation of 

 the specimens obtained. 



