29 125 
Arca (Scapharca) dichotoma, Desh. 
(Pl. 1, Fig. 3—4). 
Arca dicholoma, Drsuayes, Conchyl. de Vile de la Réunion (Bourbon), 1863, p. 22, pl. 3, figs. 18—19. 
— (Barbatia?) dichotoma, DesHayres, MAnrin1 u. CHEMNITZ, Conchyl. Cabin., VIII, 2 Abth., p. 29, pl. 8, 
figs. 7—8. 
W. of Koh Chuen, soft clay and mud (‘/2). S. of Koh Tulu, 10 fathoms, clay 
mixed with sand (‘/1)._ Between Koh Kahdat and Koh Kut, 6 fathoms, clay mixed 
with sand ('/2). W. of Koh Kut, 80 fathoms, sand and mud ('/2). 
Long. 7—17 mm. 
Distribution: — Bourbon (DESHAYES). 
Professor Ep. v. Martens, to whom I sent specimens from Koh Kut, has kindly 
determined these as A. dichotoma, Desh. This species differs, however, in several 
respects from the specimens from the Gulf of Siam, which have 25 ribs, all of 
which (except the last two at the anterior end) are divided by a narrow median 
groove into two ribs of about the same breadth, and covered with regularly-arranged 
small nodules. As the median groove does not begin quite at the umbo, specimens 
measuring, for example, about 5mm. in height scarcely have the ribs at all divided. 
The interstices between the double ribs are about as broad as the ribs and bear 
close-set transverse lines. The keel, mentioned by Desnayrs, which divides the 
median part from the posterior end, and which, in his specimens bears four undivided 
ribs, does not occur in the Siam specimens; whether this is owing to individual 
or specific character, may be known later; at present I shall name the form from 
the Gulf of Siam, which we are here considering, Arca dichotoma, Desh. var. gratiosa. 
Arca (Scapharca) Sabinae, Morl. 
Arca (Anadara) Sabine, L. Mortet, Journ. de Conchyl., vol. 37, 1889, p. 189, pl. 8, fig. 6. 
(Scapharca, — - Crosse et Fiscuer, Journ. de Conchyl., vol. 37, 1889, p. 292. 
a = — — P. Fiscuer, Catal. d. Moll. de lIndo-Chine, p. 216. 
— (Anadara) — — Mission Pavie Indo Chine, Etudes diverses, III, 1904, p. 378, pl. 22, 
fig. 6. 
— (Cunearca) — -- Lamy, Journ. de Conchyl., vol. 55, 1907, p. 276. 
Coast of Koh Chick (!%/2). W. of Koh Kut, 15 fathoms, mud ('/z). Between 
Koh Kut and Koh Kahdat, 10 fathoms, shells (!/2). Strand of Koh Kahdat ('2). Phu 
Quok in the Gulf of Siam (MorterT). 
Long. 9—17 mm. 
Distribution: — Ceylon. — Annam (Cochin China). 
The shells from the Gulf of Siam are rather inequilateral, and not so regularly 
formed as that figured by Morter; they agree most closely with the description 
which Crosse & Fiscuer give of the individuals from Annam. 
Arca (Scapharca) gubernaculum, Reeve. 
Arca gubernaculum, ReEye, Proc. Zool. Soc. London, XII, 1844, p. 40. 
= = = Conchol. icon., IH, Arca, Sp. 14. 
= — = Ene. A. Smiru, Lamellibranchiata of the Challenger Exp.. p. 266, 
