500 A. j. jukes-browne's supplementary
forms which are not uncommon among the Cambridge Pleuroto- marice. They are easily distinguishable from casts of P. Rouxi or P. Rhodani by their flat or slightly concave base.
In the abundance of its Pleurotomariai the Cambridge Greensand resembles the continental Gault Superieur ; and it is remarkable also that most of the species appear to be identical.
Pleurotomaria Itieriana ? Pict. and Ex. PI. XXI. fig. 2.
Casts of a more elevated species are often found, having 3 or 4 whorls ornamented with a spiral rib corresponding to the sinus- band ; the base is flattened ; and the general form resembles that of P. Itieriana, Pict. and Ex. pi. 22. fig 3. The same fossil also occurs in the Gault of Folkestone ; but I have never seen it named. I also possess what I believe to be a unique specimen of a reversed variety, apparently belonging to this species ; the whorls are more separated, as is frequently the case in sinistral forms, see PI. XXI. fig. 2.
LAMELLIBEANCHIATA.
Pecten aptiensis, D'Orb.
i Pecten inter striatus, D'Orb. Pal. Pr. vol. iii. pi. ccccxxxiii. figs.
1-5 ; and Pict. & Roux, Gres Verts, p. 516, pi. xlvi. fig. 4 ; subse- quently named
Pecten aptiensis, D'Orb.
Pecten Dutemplei, D'Orb. Pal. Fr. pi. ccccxxxiii. figs. 10-14 ; and Pictet & Eoux, Gres Verts, p. 516, pi. 46. fig. 4.
Pecten Barretti, Seeley, Ann. & Mag. N. H. ser. 3, vol. vii. p. 118, pi. vi. fig. 1.
I cannot help thinking that these three forms are much more nearly related to one another than has hitherto been supposed.
All three are about the same size, have nearly the same number of ribs, and are characterized by similar interstitial striations.
Of P. Barretti Mr. Seeley says, "the left valve is ornamented with some 48 radiating, rather depressed ribs, of which about a dozen are subordinate, the remainder nearly equal ; all are imbri- cated. The interstitial spaces are very finely striated, those in the centre longitudinally, and those of the sides obliquely, reminding one of P. aptiensis. On the flat valve the ribs appear less promi- nent and more numerous." I cannot see that it has any resemblance to Pecten Espaillaci ; nor do I think the latter existed with it, the specimen so named in the Woodwardian Museum being only a large individual of P. Barretti preserved in a lighter-coloured phosphate. There is no doubt, however, of its close affinity with P. aptiensis ; indeed it becomes a matter of doubt whether the differences are sufficiently great to justify its being considered a separate species.
The upper and lower valves of P. aptiensis differ similarly in the number of ribs : D'Orbigny gives 20 to 23 as the number on the upper (convex) valve, and 46 on the lower; Pictet and Eoux,
