114 18 
Ch. I cannot agree with Epc. Smira when he says! that “The few ribs towards the 
middle being duplicate, as stated by Reeve, is an individual rather than a specific 
character;” in all the specimens from the Gulf of Siam the majority of the ribs in 
the middle towards the hinder extremity are duplicate. As mentioned in connec- 
tion with A. complanata, Ch., I have unfortunately not been able to take into full 
consideration LAamy’s work on the genus Arca, as my investigations upon this genus 
were ended when the work in question was published. 
Arca (Barbatia) parva, Sow. 
Byssoarca parva, Sowerby, Proc. Zool. Soc. London, 1833, p. 19. 
Arca — _ Reeve, Conchol. icon., I], Arca, Sp. 119. 
— (Barbatia) parva, Reeve, Martini u. CHEMNITZ, Conchyl. Cabin., VII, 2 Abth., p. 197, pl. 47, fig 
= - -- Sowersy, Lamy, Journ. de Conchyl., vol. 55, 1907, p. 51. 
Barbatia attenuata, DuNKER i. 1. Museum Godeffroy. Catalog IV, 1869, p.114 — Catalog V, 1874, p. 173. 
West coast of Koh Chang, 10 fathoms, mud (1). Koh Mak, sand coast (1/2). 
Long. 15—21 mm. 
Distribution: — Persian Gulf, Djibouti, Gulf of Suez, Madagascar, — Tahiti, 
Paumotu Isl., Ducie’s Isl. 
Arca (Barbatia) Siamensis, n. sp. 
(Pl. 1, Figs. 16—17). 5 
This shell is rather thin and ventricose, oblong rectangular in outline, and 
the posterior half is swollen from the umbones obliquely downwards towards the 
posterior part of the ventral side. The valves are somewhat obliquely twisted, 
and the left valve projects, having its lower edge below that of the right valve. 
The anterior end is rounded and passes evenly into the ventral side; the 
posterior end, which is much higher, is rather straightly truncate and slopes 
obliquely downwards towards the ventral side, with which it makes a rounded 
angle; the ventral side is somewhat compressed in the middle. The surface of the 
valves is closely set with about 48 flattened, smooth, radiating ribs, which are 
provided with a longitudinal furrow towards the anterior end. The interstices 
between the ribs are narrower than the ribs. From the umbones and down to 
the middle of the ventral side there is a depression of the valves. The upper 
margin is straight and rises sharply towards the posterior end. The umbones 
are situated in the anterior third of the shell. The area is very narrow and long. 
The colour is white and the hinder margin of each valve is dark-coloured. The sur- 
face is covered by a thin, brown epidermis which in the interstices between the 
ribs is set with small scales or bristles. There are about 47 hinge-teeth. The 
interior of the valves is white, and the outer ribs project beyond the margin so 
that the latter becomes dentate. 
' Report on the zoolog. collections made during the voyage of H. M.S. ‘Alert, p. 110. 
