BIVALVIA. 87 
Since my Plate was engraved, I have seen in Mr. Prestwich’s cabinet a small Arca 
found by himself at Shapley Heath, and mentioned in the ‘Journ. of the Geol. Soc.,’ 
1847, p. 390, as an undescribed species. This shell strongly resembles the above, and 
as Mr. Prestwich considers the deposit in which it is found as belonging to the Brackle- 
sham series, it must, for the present at least, have the same name: it does not, however, 
satisfactorily conform to the characters of the Barton shell, neither to the specimens found 
at Bracklesham ; in those shells the pedal region is decidedly the larger of the two, and is 
longer and more elevated ; the whole shell is also shorter and more tumid. ‘The Shapley 
Heath specimens are comparatively longer, and the’pedal side is the shorter ; the hinge-area 
also appears to have fewer and larger denticles, and the exterior, so far as can be observed, 
is quite smooth. I feel unwilling to make another species of it from want of better 
materials, but I think it must at least be considered as a variety. 
11. Arca Lyzti1? Deshayes. Tab. XV, figs. 12, a, 4, and 13, a, 0. 
Arca LYELLI. Desh. Coq. Foss. des Env. de Par., t. i, p. 200, pl. 34, figs. 9 —11, 
1824. 2 
— — Id. An. sans Vert. du Bass. de Par., t. i, p. 873, 1859. 
Spec. Char. A. testa elongata, subcylindraced, gibbosuld, inequilaterali, plus minusve 
wrreguart, sulcatd aut radiatim costulatd et concentrice squamoso-lamellosa vel tuberculosa ; 
siphoni-regione longiore angulatd aut oblique truncata; umbonibus minimis, depressis, 
obliquis ; ared connexis angustd, dentibus mediantis irregularibus ; marginibus crenulatis. 
Shell transversely elongate, subcylindrical, slightly tumid, inequilateral, more or less 
irregular in outline, covered with radiating and rounded ridges decussated by lamellated 
lines of growth; siphonal region the longer, and obliquely truncated ; umbones depressed : 
area for connector small, narrow ; margins crenulated. 
Length, sths of an inch. 
Localities. Barton (Edwards), Colwell (H. H. Wood), 
This species seems to be rare in our Eocene beds. A few specimens from Barton 
are in Mr. Edwards’s cabinet, and the Rev. H. H. Wood has kindly sent me a specimen 
from Colwell Bay. 
The shell from the French beds, to which this is with some slight doubt referred, is 
said to be variable; and M. Deshayes has proposed two new species, 4. contorta and 
A. lamellosa, which he thinks, however, may be ultimately united to 4. Lye/li, and I am 
inclined to the same opinion. ‘The shell represented by fig. 13 of our Plate, which I at 
first imagined to be distinct, may perhaps be referred to /amed/osa, and that by fig. 12 to 
contorta ; these all so closely accord with 4. clathrata, that I am doubtful whether the 
‘Touraine shell be anything more than a variety induced by difference of conditions. Among 
the few English specimens that I have seen, there is considerable variation ; im one the 
» dental area has only a few large teeth, in another of the same size this margin is studded 
