154 58 



Among the Pec/e;i-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, Inhanibane (SE. 

 Africa). — Philippines, China, Japan, Rlitang (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 dilTerent names has been given to it. 

 Edg. A. Smith^ 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. Martens-' distinguishes, but scarcely correctlj', between P. porphyreus, 

 Chemn. (= P. senator his. 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 thai P. splendiduhis. Reeve, belongs 

 to the variety-series of P. senatorius. 



Pecten rugosus, Sow. 



(PI. Ill, Figs. 1-2). 



Pecten rujjosus, Sowf.rby, Thesaurus Coiichyl., I, Pecten, p. Oö, No. 64, pi. 19, fig. 22(5. 



— — — Reeve, Conchol. icon., VIU, Pecten, Sp. 144. 



— — — Martini ii. Chemnitz, Conchyl. Cabin.. VII, 2 Abtli., p. 2,32, pi. (il, fig. 8. 



' Zoology of the voyage of H. M. S. Challenger. Report on the Lamellibranchiata, p. 300. 



- 1 have at ray disposal several specimens from tlie Gulf of Siam which correspond exactly with 

 the figure in Savigny, Iconographie d. Mollusques de l'Egypte, pi. 13, fig. 5, 2 of "Pecten australis, Sowerby' 

 (IssEi., 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 Linneaii Soc, Zoology, 

 vol.21, 1887, p. 202. — Rumphius Gedenkhoek, 1002. Mollusken, p. 12(i il8). 



* Bull, du Museum d'Hist. Nat. Paris, 1904, p. 306. 



