88 EOCENE MOLLUSCA. 



with double the quantity of denticles ; in some the margin is crenulated all round, in others 

 the central portion appears to be free from crenulations, and in A. contorta the margin is 

 said to be smooth ; this variation is perhaps dependent upon the state of preservation of 

 the specimens. The surface of our shell is covered with rounded rays more or less broad 

 or numerous, and these rays generally project at the margin, particularly on the siphoni- 

 1 ateral region ; this is more especially the case in the shell fig. 12, where the diagonal or 

 carina! ridge is prominent, and the rays are more distinctly lamellated. The position of the 

 umbo is not a permanent character, some specimens being more inequilateral than others. 



12. Arca modioliformis, Deshayes. Tab. XIV, fig. 5, a, b. 



Arca modioliformis. Desk. Coq. Foss. des Env. de Par., t. i, p. 214, pi. 32, figs. 5, 6, 1824. 



— — Id. An. sans Vert, du Bass, de Par., t. i, p. 896, 1858. 



— — Potiez et Mich. Gal. de Douai, t. ii, p. Ill, No. 16, 1844. 



Spec. Char. A. testa elongato-obtiqud, ovatd vel irregulariter trapeziformi, valde 

 inaquilaterali, modioUformi ; radiatim striata, striis in siphoni-regione depresses, undnlatis 

 distantioribus ; cardine in medio edentulo. 



Shell elongately oblique, ovate or irregularly trapezoidal, very inequilateral, radiately 

 striated, the striae on the siphonal region somewhat undulating and distant; hinge-line 

 without denticles in the centre, with a few only at each extremity. 



Length, 1 inch ; height, ^ths of an inch. 



Localities. Stubbington {Edwards). 



France, Cuise-Lamothe, Valmondois [Deshayes). 



This is apparently a tolerably well-marked species. I have seen only three British 

 specimens ; these correspond so well with the Continental shell, that I think there will be 

 no dissent from the identification. 



M. Deshayes gives two varieties of his species in his first work, but in his more recent 

 one he has separated what were formerly included under the above name. Our shell 

 appears better to agree in form and in the exterior ornament with what he has called 

 A. Bigaultiana ; but the ligamental area is narrower, neither has the British fossil so many 

 denticles. Old shells will often have the area of connexus enlarged, but the central 

 portion of the dental area in that case has the denticles obliterated, somewhat after the 

 manner of those in Pectunculus. 



13. Arca nitens, J. Sowerby. Tab. XV, fig. 5 a, b. 



Arca nitens. J. Sow. Trans. Geol. Soc., 2nd ser., vol. v, p. 136, pi. 8, fig. 9, 1834. 



— — Prestwich. Geol. Journ., 1847, p. 401. 



Spec. Char. A. testa elongatd, obliqud, subtrapezoidali, convexd, tumidd, lavigatd 



