154 58 
Among the Pecten-species this was the one which was taken most frequently; 
from Koh Rin to Koh Kut in 5—38 fathoms, on a bottom of mud and shells. 
Long. 4—67 mm. 
Distribution: — As the question regarding the variations in form of the 
present species has not yet been quite settled, its distribution cannot be given with 
any certainty, but the following localities may doubtless be regarded as certain: — 
Salanga, Mergui Archip., Madras, Gulf of Manaar, Bombay, Karachi, Persian Gulf, 
Maldive Archip., Gulf of Aden, Red Sea, Gulf of Suez, Mauritius, Inhambane (SE. 
Africa). — Philippines, China, Japan, Blitang (post-tertiary), Moluccas, Torres Strait, 
Mast Head Reef (Queensland), New Caledonia. 
The considerable variations in form, to which the present species is subject, 
have caused that an enormous number of different names has been given to it. 
Epa. A. Smirn! has enumerated a large number of these forms, most of which can 
scarcely be regarded as varieties. The large quantity of material brought home 
from the Gulf of Siam shows plainly the variability of the species, both as regards 
sculpture and colour. The form porphyreus, Chemn.,* occurs most frequently; there 
are developed upon it, when the individuals have reached a certain size, smaller ribs 
on each side of the main ribs, which thereby become tripartite; the whole surface 
is closely covered with scale-sculpture; this form strongly recalls P. asperrimus, 
Lam. (P. australis, Sow.). I have compared CHEMNITz’s original specimen of P. por- 
phyreus, which is in the Zoological Museum of the University of Copenhagen, with 
the specimens from the Gulf of Siam; they correspond in all respects with the 
latter. E.v. Marrens® distinguishes, but scarcely correctly, between P. porphyreus, 
Chemn. (= P. senatorius, Reeve, non Chemn.), and P. senatorius, Chemn.; these forms 
cannot be kept distinct from each other, as transitional forms occur. In the Gulf 
of Siam are found specimens, some with quite smooth ribs, and others with strong 
scale-sculpture on all the’ribs. Bavay* thinks that P. splendidulus, Reeve, belongs 
to the variety-series of P. senalorius. 
Pecten rugosus, Sow. 
(PI. III, Figs. 1—2). 
Pecten rugosus, Sowrersy, Thesaurus Conchyl., I, Peclen, p. 66, No. 64, pl. 19, fig. 226. 
— Reeve, Conchol. icon., VII, Pecten, Sp. 144. 
_ Martini u. CHemnitz, Conehyl. Cabin.. VU, 2 Abth., p. 232, pl. 61, fig. 8. 
1 Zoology of the voyage of H. M.S. Challenger. Report on the Lamellibranchiata, p. 300. 
’ | have at my disposal several specimens from the Gulf of Siam which correspond exactly with 
the figure in Saviany, Iconographie d. Mollusques de l’Egypte, pl. 13, fig. 5, 2 of “Pecten australis, Sowerby’ 
IsseL, Malacol. d. Mar Rosso, p. 372). In my opinion this figure represents most closely a very well- 
preserved individual of P. senatorius, Ch. var, porphyreus, Ch. 
’ Die Mollusken d. Maskarenen u. Seychellen, 1880, p. 138. — Journ. of the Linnean Soc., Zoology, 
vol. 21, 1887, p. 202. Rumphius Gedenkboek, 1902. Mollusken, p. 126 (18). 
‘ Bull. du Museum d’Hist. Nat. Paris, 1904, p. 366. 
