124 98 
In Reeve’s diagnosis, and in the descriptions of A. clathrata given by later 
authors, I do not find any mention of several distinctive characters, peculiar to this 
species. Thus, in the specimens from the Gulf of Siam, the sculpture is different 
on the two valves; on the right valve it is only the 8—10, most anterior ribs which 
are set with regularly-arranged, small nodules which gradually disappear almost 
completely from the rest of the ribs; also, on the right valve, all the ribs are 
somewhat flatter than are those on the left valve. The latter is more strongly 
sculptured, and on the 18—20 most anterior ribs the small nodules occur regularly 
and distinctly, like strings of beads; but they are frequently absent from the most 
posterior ribs. Upon both the umbones there is a distinct median groove, which 
occurs in all the specimens, from the smallest to the largest, and which affords a 
good distinguishing feature by means of which they may be separated from forms 
nearly related to them. A. H. Cooke! considers A. rotundicostata, Reeve, to be 
synonymous with the present species. I believe the same is the case with A. Troscheli, 
Dunker.? Epa. Smiru® doubts ReEve’s statement that the epidermis is “very finely 
bristly between the ribs;” but among the specimens from the Gulf of Siam there 
are several in which this can be seen, and the bristles in such cases, are parti- 
cularly strongly developed between the most posterior ribs. Specimens, however, 
also occur, with a perfectly well preserved epidermis, from which these bristles 
are entirely absent. As the present species is provided with a byssus, it cannot be 
referred to the sub-genus Anadara, but must belong to Scapherca.! The Siam 
=xpedition brought home several specimens in spirit which show a well-developed 
byssus. 
Arca (Scapharca) consociata, E. Sm. 
Arca (Scapharca?) consociata, Epg. A. Smrrn, Challenger Exped. Report on the Lamellibranchiata, 1885, 
p. 266, pl. 17, fig. 7. 
— — — — Martini u. Cuemnirz, Conchyl. Cabin., VII, 2 Abth., p. 221. 
— consociata, Enc. A. Smirn, Ann. and Mag. Nat. Hist, 7 Ser., vol. 14, 1904, p. 12. 
W. of Koh Chuen, soft clay (**/2 young specimens). S. of Koh Samit, 30 fathoms, 
mud (‘/2). Off Tung Kaben, 6 fathoms, clay mixed with sand (‘/z). Sound of Koh 
Chang, 4—5 fathoms, soft clay (8 + */2). West coast of Koh Chang, 10 fathoms, 
mud (1). E. of Koh Mak, 20 fathoms, mud (2). Between Koh Kahdat and Koh 
Kut, 6 fathoms, clay mixed with sand (*/2). 
Long. 2—38 mm. (long. 38, alt. 27. — long. 26, alt. 18). 
Distribution: — Off Chedubar (Aracan coast, 20—30 fathoms, SmirH). — 
Arafura Sea (SMITH). 
As may be seen from the measurements given above, Epc. Smiru is right in 
believing that his specimen with a length of 12°55 mm., collected during the Chal- 
lenger Expedition, is not full-grown. 
1 Annals and Magazine of Nat. Hist. 5 Ser., vol. 18, 1886, p. 95. 
2 Index Molluscor. mar. Japonici, p. 234, pl. 14, figs. 14—15. 
8 Proe. Zool. Soe. London, 1891, p. 482. 
! pq. A. Smrru, who, in the Challenger Report, 1885, refers A. clalhrata to the sub-genus Scapharca, 
in 1891 (loc. cit. p. 432) had changed his opinion, and placed it under Anadara. 
