162 66 
Long. 74 mm., alt. 54 mm. 
— 59 mm., — 64 mm. 
Distribution: — China Sea, Nagasaki, Yaeyama (?). 
A few of the specimens collected were attached to Mangrove-roots. Future 
comparative investigations will doubtless prove that O. rivularis, Gould (Lischke), 
is only a form of a previously known species,’ but I have thought it right to 
stop my researches at rivularis (with which the specimens from Siam entirely 
correspond), as a large quantity of material is required to arrive at a reliable con- 
clusion. The whole group (with thin, not folded shell-margin), which includes 
O. mytiloides, Lam., nigromarginata, Sow., tuberculata, Lam. (= tubercularis, Reeve), 
Paulucciae, Crosse (= palmipes, Sow.), the collective species arborea, Ch. and para- 
sitica, Gm. (which constantly reappear), and many more, must, on account of their 
great variability, and the hitherto very uncritical and superficial treatment to which 
they have been subjected, be carefully investigated and may, no doubt, be con- , 
siderably reduced. 
Ostrea Paulucciae, Crosse. 
Ostrea Pauluccice, Crosse, Journ. de Conchyl., vol. 17, 1869, p. 188; vol. 18, 1870, p. 108, pl. 2, fig. 5. 
Ostrea palmipes, SowERBY, REEVE, Conchol. icon., vol. 18, 1871, Ostreea, Sp. 56. 
S. of Koh Tulu, 10 fathoms, sand mixed with mud (*/2). Sound of Koh Chang, 
5 fathoms, soft clay (1). S. of Koh Kahdat, 8--10 fathoms, mud (many ycoung odd 
valves). Between Koh Kahdat and Koh Kut, 6 fathoms, sand mixed with mud (*/:2). 
W. of Koh Kut, 15 fathoms, mud (?/2). 
Long. 28 mm., alt. 20 mm. 
Distribution: -- Annam, China, Philippines. 
At Koh Chang (in 5 fathoms, on a soft muddy bottom) a smaller specimen 
was obtained (long. 15 mm., alt. 18 mm.) which seems to me to agree in all points 
with O. pes-tigris, Hanl.;* it is doubtful whether this form also is entitled: to rank 
as a species. 
The specimens from the Gulf of Siam do not correspond exactly with Crosse’s 
description as regards colour, as in almost all cases they are white with a few 
lilac rays or shades. 
Note. In Morcn’s Catalogus conchyl. qu reliq. comes de Yoldi, II, p. 62, 
No, 783, there is recorded an Ostrea siamensis, Yoldi, from “Singapuhra,” as it is, 
however, not described, either there or anywhere else, I cannot state anything 
regarding that form. 
1} Dunker (Index moll. mar. Japon., p. 250) refers it to O. arborea, Ch. 
’ Proc. Zool. Soc. London, XIII, 1845, p.107. Taken by Cumina at Luzon, Philippines. REEve, 
Conchol. icon., XVIII, Sp. 78. Sowrrsy (Append. to marine shells of South Africa, 1897, p. 28) is of 
opinion that O. pes-tigris (REEve, fig. 78) is synonymous with O. lacerata, Hanl. (= O.lacerans, REEVE). 
