86 EOCENE MOLLUSCA. 



10. Arca laevigata, Caillat. Tab. XV, fig. 8, a, b. 



Arca laevigata. Caillat. Desc. des quelq. Coq. Nouv., p. 4, pi. 2, fig. 7, 1834. 



— — Nyst. Tabl. Syn. des Arches, p. 40, No. 212, 1849. 



— — TJOrb. Prod, de Paleout., t. ii, p. 390, No. 1059, 1850. 



— — Pictet. Traite de Paleont., t. iii, p. 551, 1855. 



— — Desk. An. sans Vert, du Bass, de Par., t. i, p. 905, pi. 68, figs. 23—26, 



1858. 



— elegans. £. Wood. Lond. Geol. Journ., p. 3, 1846. 



— pu.etenuis. Charlesworth. MS. Nat. Hist. Soc. Illust. 



Spec. Char. A. testa minutd, glabra, tumidd, ovato-subquadrangulari vel subtra- 

 peziformi, subaquilaterali ; pedi-regione late semicirculari ; siphoni-regione paulo minore, 

 angulatd vel oblique truncatd ; umbonibus acutis, dislantibus ; margine integro, denlibus 

 in medio interruptis ; fossuld in area cardinali excavatd. 



Shell small, glossy, ovately quadrate or slightly trapeziform ; subequilateral, sub- 

 equivalve, tumid ; pedilateral margin rounded ; siphonilateral truncated or angulated ; 

 beaks distant ; margins smooth ; triangular depression in cardinal area. 



Length, ^th inch ; height, ^th of an inch. 



Localities. Barton, Bracklesham {Edwards), Isle of Wight [Charlesworth). 

 France. Grignon, &c, Calcaire grossier {Desh.). 



This elegant little shell is by no means rare in England, and specimens present 

 considerable variation. 



I have obtained it also from a small patch of the so-called Upper Marine, which 

 intervenes between the true freshwater deposits at Ilordle. 



There is a peculiarity about this and one or two other species hitherto included in the 

 genus Arca which will entitle them to be placed in a distinct section, perhaps to be 

 regarded as forming a distinct genus ; they present the same difference from Arca that 

 Limopsis does from Fectunculus, having a portion of connexus placed in a triangular pit 

 immediately beneath the umbo. 



M. Deshayes has figured and described two species with this peculiarity in the 

 connector, viz., A. laevigata and A. effossa, the latter differing from the former in having 

 the exterior surface more distinctly cancellated, while the former is described as being 

 quite smooth, as the name imports. Our little shell corresponds in outline, and pretty 

 well so in magnitude and relative proportions, with both these species, but it does not 

 correctly agree with either in the ornamentation, except that there are a few more promi- 

 nent rays over the angular ridge on the siphonal region ; it has the exterior cancellated, 

 though in a fainter or minor degree than A. effossa, but it is not smooth in perfect 

 specimens. This, M. Deshayes remarks, is the smallest known species of the genus, and it 

 is not quite equivalved, the right valve being slightly the larger of the two. It approaches 

 closely to Trigonoccelia, and might be called Trigonodesma. 



