112 16 
Arca (Barbatia) decussata, SowrBsy, Martini u. CHEMNITz, Conchyl. Cabin, VIII, 2 Abth., p. 144, pl. 37, 
figs. 2—5. 
-~ -- — E. Lamy, Bull. du Mus. d’Hist. Nat. Paris, 1904, p. 273. Loc. cit. 
1906, p. 317, No. 78. 
Byssoarca velala, Sowrersy, Proc. Zool. Soc. London, vol. 1, 1833, p. 18. 
Arca _ -- REEVE, Conchol. icon., Il, Area, Sp. 79. 
Barbatia = = var., DuNKER, Novitates conchologicae, p. 124, pl. 41. 
Arca (Barbatia) velata, Sowrersy, Martini u. CHEMNITZ, Conchyl. Cabin., VII, 2 Abth., p. 207, pl. 49, 
figs. 2—3. 
The north side of Koh Mesan, 10—15 fathoms, stones (1). Koh Chick, rocky 
coast (3). North end of Koh Chang, 1 fathom, old coral blocks (38). The same 
locality, 12 fathoms, old coral blocks (4). West coast of Koh Chang, 10 fathoms, 
mud (5). Koh Sarlak (2). Lem Ngob, on piles of bridge, very low tide (2). Koh 
Lom (3). Strand of Koh Kahdat (1). Koh Mak, sand coast (1 + */2). 
“Sinum Siamensem habitare dicitur” (Barbatia velata, Sow. var., Dunker). Islands 
in the Gulf of Siam (A. complanata Ch., L. Morlet, loc. cit.). 
Long. 8—55 mm. 
Distribution: — Singapore,' Salanga, Mergui Archipel, Nicobar Isls., Madras, 
Ceylon, Persian Gulf, Gulf of Aden, Red Sea, Gulf of Suez and Akaba, Seychelles, 
Madagascar, Mascarenes. — Poulo Condor, China, Japan, Philippines, Celebes, Torres 
Strait, New Caledonia, Viti Isls., Funafuti (Ellice Isl.), Paumotu Isl., Society Isls., 
New Zealand, Galapagos Isls. (San Thomé in the Gulf of Guinea, Hoyle ®). 
On the basis of the material at hand, I have united Arca decussata, Sow. 
with A. velata, Sow. into one species, as it is not possible to keep these forms 
distinct, A. decussata merging by easy stages into A. velata, while the specific charac- 
ters which have been pointed out, are not constant. In Pl. 1, Figs. 5—10 I have 
figured several forms which show this transition better than it could be shown by 
any description. It was undoubtedly owing only to the lack of a larger quantity 
of material that Sowersy established two species upon specimens taken in the 
same localities (“Lord Hood’s and Chain Islands, Pacific Ocean, attached to shells. 
Cuming’’).? The literature gives indications of the great uncertainty which authors 
show in the determination of the highly variable Barbatia-forms; while a great 
number of species: — A. foliata, Forskal,’ revelata, Dsh., Petersi, Dkr., sculptilis, Reeve 
and many others need to be known in more complete deyelopment- and varia- 
tion-series before their distinctness can be guaranteed. The reported occurrences of 
the West-Indian species Arca Heblingi, Ch. (candida, Gmel.) in Asiatic waters are 
without doubt due to erroneous determinations, and all the specimens in question 
should evidently be referred to A. complanata; nor is A. nivea, Ch. sufficiently defined 
as a species, and it needs revision. PAGENSTECHER however, goes too far,® when 
) Taken by the Danish Expedition to Siam, four specimens, in 2--3 fathoms, coral-reef. 
* tn spite of Carmnirz’s naming Guinea as the home of his species A. complanala, an undoubtedly 
incorrect statement, | have not hesitated in referring the forms we are here considering to that species. 
Sy. Marrens, Vorderasiat. Conchylien 1874, p. 108, pl. 9, fig. 58. 
4 Ep. Lamy, Journ. de Conchyl., vol. 52, 1904, p. 140. Bullet. du Mus. d’Hist. Natur., 1904, p. 272. 
Jeber die yon R. KossMANN am Rothen Meere gesamm. Mollusken. Lpzg. 1877, p. 35. 
