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 4. 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- 
lateral region ; this is more especially the case in the shell fig. 12, where the diagonal or 
carinal 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 moprioLirorMis, Deshayes. ‘Tab. XIV, fig. 5, a, 6. 
ARCA MODIOLIFORMIS. Desh. Coq. Foss. des Env. de Par., t.i, p. 214, pl. 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. 111, No. 16, 1844. 
Spec. Char. A. testa elongato-obliqud, ovatd vel irregulariter trapeziformi, valde 
nequilaterali, modioliformi ; radiatim striata, striis in siphoni-regione depressis, undulatis 
distantioribus ; cardine in medio edentulo. 
Shell elongately oblique, ovate or irregularly trapezoidal, very imequilateral, radiately 
striated, the striz 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, iths of an inch. 
Localities. Stubbington (Ldwards). 
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. Rigaultiana ; 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, 4. 
Arca NITENS, J. Sow. Trans. Geol. Soc., 2nd ser., vol. v, p. 136, pl. 8, fig. 9, 1834. 
— — Prestwich. Geol. Journ., 1847, p. 401. 
Spec. Char. A. testd elongata, obliqud, subtrapezoidali, convexd, tumidd, levigatd 
