87 183 
The strand off the mangrove at the station on Koh Chang (1). Off Koh Kut 
5 fathoms (2). Coast of Koh Kong (6). 
Long. 3—7 mm. 
Distribution: — Ceylon, Aden (SHopLAND). — South of New Guinea, Torres 
Strait. 
> 
Montacuta (Tellimya) triangularis, A. Ad. 
Pythina triangularis, A. AbAms, Proc. Zool. Soc. London, XXIV, 1856, p. 47, No. 5. 
— — —_ EpaG. Smiru, Annals and Mag. of Nat. Hist., vol. VIII, 1891, p. 231, No. 8. 
Mactra nucleus, Rerve, Conchol. icon., VII, Mactra, Sp. 102 (non M. nucleus, Conrav, Journ. Acad. 
Nat. Se. Philadelphia, VI, part 2, 1830, p. 258, No. 3). 
Koh Kahdat (‘/2). S. of Koh Kahdat, 8—10 fathoms, mud (4/2). N. of Koh 
Kut, 10 fathoms, mud (1/2). 
Long. 5—7 mm. 
Distribution: — Manilla (A. ApAms),. 
Kellya subsinuata, Lischke, resembles the present species very much in form. 
Montacuta (Tellimya) variabilis n. sp. 
(Pl. Ill, Figs. 30—32). 
This species is triangular in form, dully white, semitransparent, rather thin 
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 embryonic 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 which are placed 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 Pl. Ill, Figs. 30—31 the two mosi divergent forms, which limit the variation of 
the specimens obtained. 
