92 EOCENE MOLLUSCA. 



17. Arca tumescens, Edwards, MS. Tab. XV, fig. 1, a, b. 



Spec. Char. A. testa mcdiocri, subquadrangulari, sub-inaqtdlaterali, aquivalvi, gibbosd 

 vel tumidd; radiatim tenuissime striata et concentrice decussatd, politd; siphoni-regione 

 longiore, vix latiore ; marginibus dorsali et ventrali subparallelis, umbonibus approximatis, 

 depresses ; area 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, £ths ; height, §ths of an inch. 



Localities. Clarendon, Brook {Edwards). 



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 striae 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. lactea. 



18. Aiica Websteri, Forbes. Tab. XV, fig. 11, a, b. 



Arca Websteri. Forbes. Mem. Geol. Surv., 1856, p. 150, pi. 3, fig. 8. 



Spec. Char. " T. parvd, ovato-oblongd, modioliformi, transversim sulcata, radiatim. 

 striata, ant ice angustiori, postice Jatiori, effusd, sub-carinatd ; striis posticis elevatis, acutis, 

 striis ceteris obscuris ; carina rotundatd ; cardine intemipto, dentibus prominentibus, 

 distantibas" 



" A small, depressed, modioliform shell, transversely sulcated and with radiating striae ; 

 the anterior narrow, the posterior part spread out and somewhat carinate ; the posterior 

 striae 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." — Morris. 



Length, |ths of an inch ; height, half the length. 



Loccditg. "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 



