1890.] MARINE MOLLUSCA OF ST. HELENA. 305 



LiTHODOMUS BI-EXCAVATUS, RcCVe ? 



L. bi-excavatus. Reeve, Conch. Icon. pi. 4. figs. 22 a, b. 



Hab. St. Thomas, West Indies. 



The shells obtained at St. Helena by Mr. Melliss and named L. 

 lithophagus, Linn., by Jeffreys, do not belong to that species. They 

 may be considered a variety of L. bi-excavatus, in which the two 

 depressions are not quite so distinct as in the type. The chalky 

 incrustation which invests them has a more openly reticulated or 

 spongy appearance at the posterior end. 



Arca sanct^-helen^. (Plate XXII. figs. 8-8 b.) 



Testa oblonga, crassa, albida, rufo-strigata et variegata, inferne 

 huud Mans ; valvce solidte, antice oblique curvatce,postice paulo 

 Jatiores, curvatim truncates, radiatim costatce, lineisque tenui- 

 bus concentricis et transversis decussatcB ; costce inrequalps, 

 subnodosce, anteriores et posteriores crasscB, mediance tenuiores ; 

 pagina interna alba, ad marginem saturate purpureo-fusca postice 

 fortiter dentata ; umbones remoti, incurvati, prominentes ; liya- 

 menti area lata, concava ; ligainentum subrAomboidale, fuscum, 

 sutcis paucis sculptum. 

 Longit. 66 millim., diam. 47, alt. 36. 



This is a strong, heavy species, belonging to the same group as 

 A. noce, A. navicularis, and the like. 



It is more solid than either of the above-named species, has the 

 posterior end unsinuated, and the margins of the valves are peculiarly 

 dentate posteriorly, and, when closed, interlock like the valves of 

 Ostrea crista-galli and some others. A few of the ribs near the 

 posterior angle of the valves are very large and strong, and separated 

 by very deep grooves. 



The form is rather like that of ^. subquadrangula of Dunker, but 

 the posterior end is not so truncate and the costae are different. 



A.RCA (Acar) domingensis, Lamarck. 



Hab. West Indies, Cape Verde Islands, S. Africa, Red Sea, Indian 

 Ocean, South Pacific Ocean, Japan, Australia, &c. (Lischke). 



Pinna rugosa, Sowerby. 



Pinna rugosa, Sowerby, Reeve, Conch. Icon. pi. 26. fig. 50. 



Hab. Isle of Rey, Bay of Panama (Cuming). 



Capt. Turton remarks as follows respecting the single broken valve 

 obtained: — " It measured when perfect 19 inches, but I cannot be 

 sure of this identical shell being an island one, as I bought it ; but 

 I have seen another just like it, 16 inches long, which was fished 

 up alive and bought by another oflScer before I heard of it, so this 

 is probably an island one too." 



Is there some mistake here, or does this species really occur at 

 Panama ? 



