55 



Subgenus Bathyarca K obéit. 



1891. Bathyarca Kobelt, in Martini Chemn. Conch.-Cab. (N. F.), VIII (2), Arca, p. 214. 

 1898. Bathyarca Dali, Trans. Wagner Free Inst. Sci. Philadelphia, III, p. 619 (as a group 



of Scapharca). 

 1907. Bathyarca Lamy, Journ. Conchyliol., LV, p. 6. 



The species of this subgenus with the génotype A. {B.) pedunculoides Scacchi generally 

 hâve a small shell and are found at great depths. Dall has published an excellent description 

 of its diaofnostic characters. 



'«3 



1. Arca [Bathyarca) anaclima Melvill. (Plate VIII, figs. 9, 10). 



1906. Arca [Bathyarca] anaclima Melvill, Proc. Zool. Soc. London, p. 794, pi. LIV, figs. 6, 6a. 

 191 1. Arca [Bathyarca) sp., Pelseneer, "Siboga" Lamellibranches (Anat.), pp. 9, 15. 



I refer a single spécimen with the right valve partly damaged and the animal of which 

 was described by Dr. P. Pelseneer as that of Arca {Bathyarca) sp., to this species with some 

 hésitation. It agrées fairly closely with the description and figures of A. [B.) anaclima from the 

 Gulf of Oman, Persian Gulf, but I hâve not been able to compare it with the type. 



The shell is small, somewhat longer than high, obliquely truncate, inequilateral, inequivalve, 

 of a dirty yellow colour and covered with a thin epidermis. The valves are sculptured with fine 

 concentric lirae, which are crossed by rather stronger lirae radiating from the umbones ; the 

 radia ting lirae are less strongly developed in the middle région, and are better marked on the 

 right than on the left valve. The umbones are minute, only slightly raised above the hinge-line, 

 slightly acute, a little remote from one another and situated a little in front of the middle. The 

 measurements of the single shell are: — Length 3.9 mm., height 3.4 mm., diameter 1.6 mm. 



Distribution : — The species, as has been noted above, was described from the Persian 

 Gulf, while the spécimen hère recorded was found in fine, yellow mud, from a depth of 1301 mètres 

 at Stat. 88 (0° 34^.6 N., ii9°8^5E.). 



2. Arca {Bathyarca) sibogai sp. nov. (Plate II, figs. i — 3). 



I give below a description of this interesting species which is not represented by complète 

 spécimens, but of which 4 valves, 3 left and i right, were dredged by the "Siboga" Expédition 

 at a depth of 1158 mètres. 



Shell very small, thin, very inflated; hinge-line somewhat shorter than the maximum length 

 of shell, which is of a rounded triangular form, with prominent prosocoelous umbones ; left valve 

 with fine, somewhat elevated, rounded, concentric lines crossed by doser, much less prominent 

 and finer radial lines ; on the right valve, as is usual in this subgenus, the radial sculpture is 

 more prominent and consists of very minute but distinct radial lines running ail over the surface 

 and producing a délicate cancellated sculpture ; cardinal area moderately wide ; umbones sub- 

 central ; surface of area longitudinally striated ; hinge with six to seven vertical teeth separated 

 by a moderately wide smooth area from five to six smaller, more oblique posterior teeth ; margin 

 of valve thin, entire. 



