92 EOCENE MOLLUSCA. 
17. Arca tumuscens, Ldwards, MS. Tab. XV, fig. 1, a, 6. 
Spec. Char. A. testi mediocri, subquadrangulari, sub-inequilaterali, equivalvi, gibbos 
vel tumidd; radiatim tenuissimé striatd et concentricé decussatd, politd; siphoni-regione 
longiore, vix latiore; marginibus dorsali et ventral subparallelis, umbonibus approximatis, 
depressis ; ared cardinali angustd. 
Shell of moderate size, subquadrangular, slightly inequilateral, equivalve, gibbous or 
tumid, finely radiated, and decussated by slender concentric lines of growth, glossy ; dorsal 
and ventral margins nearly parallel; beaks small or depressed, with a narrow ligamental 
area. 
Length, sths; height, sths of an inch. 
Localities. Clarendon, Brook (Hdwards). 
Mr. Edwards’s cabinet contains several specimens of this species, in good preservation, 
and they appear to deserve a distinct specific name. ‘The shell, in some characters, 
resembles A. impolita, but it is more tumid, more equilateral than that species, and it is 
also more quadrangular, and it is polished and glossy ; the very fine striz with which it 
is covered are scarcely visible to the unassisted eye; the lateral margins are roundedly 
angular, and the siphoni-lateral region is rather the broader or higher of the two. It is a 
handsome shell, resembling, in some slight degree, our common recent species A. laciea. 
18. Arca WessteRrI, Forbes. Tab. XV, fig. 11, a, 6. 
Arca Wrsstert. Forbes. Mem. Geol. Surv., 1856, p. 150, pl. 3, fig. 8. 
Spec. Char. “ T. parvd, ovato-oblongd, modioliformi, transversim sulcatd, radiatim 
striata, antice angustiori, postice latiori, effusd, sub-carinatd ; striis posticis elevatis, acutis, 
striis ceteris obscuris; carind rotundaté ; cardine interrupto, dentibus prominentibus, 
distantibus.” . 
«A small, depressed, modioliform shell, transversely sulcated and with radiating striae ; 
the anterior narrow, the posterior part spread out and somewhat carinate; the posterior 
strie are elevated and acute.” 
“The umbones are placed near the anterior margin, and the middle part of the car- 
dinal area is without teeth. The teeth are prominent and distant.”—WMorris. 
Length, :ths of an inch; height, half the length. 
Locality. ‘‘ From the Bembridge series.” (Morris.) 
This is a pretty little species, and appears to be confined to the Upper Eocene Deposits. 
Its principal distinctions are the form of the siphonal region and the ornament with 
which it is covered. The shell is rather tumid, and a very obtusely angular ridge or 
