86 EOCENE MOLLUSCA. 
10. Arca Lavieata, Caillat. Tab. XV, fig. 8, a, 6. 
Arca LaviGatTa. Caillat. Desc. des quelq. Coq. Nouv., p. 4, pl. 2, fig. 7, 1834. 
—_ = Nyst. Tabl. Syn. des Arches, p. 40, No. 212, 1849. 
— — D’ Orb. Prod. de Paléont., t. ii, p- 390, No. 1059, 1850. 
— — Pictet. Traité de Paléont., t. ili, p. 551, 1855. 
=— — Desh. An. sans Vert. du Bass. de Par., t. i, p. 905, pl. 68, figs. 23—26, 
1858. 
— e1Ecans. S. Wood. Lond. Geol. Journ., p. 3, 1846. 
— PR#TENUIS. Charlesworth. MS. Nat. Hist. Soc. Illust. 
Spec. Char. A. testd minutd, glabrd, tumidd, ovato-subquadrangulari vel subtra- 
pezformi, subequilaterali ; pedi-regione laté semicirculari ; siphoni-regione paulo minore, 
angulatd vel oblique truncatd ; umbonibus acutis, distantibus ; margine integro, dentibus 
in medio interruptis ; fossuld in ared 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, ith inch; hezght, 3th of an inch. 
Localities. Barton, Bracklesham (dwards), Isle of Wight (Charlesworth). 
France. Grignon, &c., Caleaire 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 Hordle. 
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 Area that 
Limopsis does from Pectunculus, 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., 4. levigata and A. effossa, the latter differmg 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 Zrzgonocelia, and might be called T'rigonodesma. 
