NOTES ON THE CAMBRIDGE GREENSAND. 501
however, reckon the number on the upper valve as 35 to 38, but have not seen the lower valve.
Pecten Dutemplei is said to have about 40 or 50 ribs on the upper valve ; " and a small simple linear rib occurs between these ribs, but not regularly." This arrangement is better shown in Pictet and Roux's figure than in that of D'Orbigny ; and the number of ribs given by the latter, viz. 70 to 80, is probably an exaggeration.
Both P. aptiensis and P. Dutemplei have the same curious oblique striation across the interspaces, and are in all other respects very like one another, the chief difference mentioned by D'Orbigny being the presence of the occasional intermediate simple rib on the latter species.
Now occasional simple ribs are also distinctly visible on some specimens of P. Barretti, especially on that previously referred to as having been mistaken for P. Espaillaci. P. Barretti, therefore, would appear to be most nearly allied to the Grault form of the species, viz. P. Dutemplei.
Mr. Seeley finds some differences in the ornamentation of the ears ; but his remarks regarding them (p. 119) are not very clear : neither does the variation seem very important ; for the combination of transverse and radiating lines is very frequent on the ears of allied species, and sometimes one set of markings, sometimes the other, stands out more prominently, according to growth and state of preservation.
All the differences above noted are merely slight variations from the earlier type of P. aptiensis, under which head I think both P. Dutemplei and P. Barretti should be ranged as varieties. The latter has been found by Mr. H. G. Fordham in the Greensand at Swanage.
Pecten Rauliniantjs ? D'Orb.
Pecten Raulinianus, D'Orb. Pal. Fr. iii. pi. ccccxxxiii. figs. 6-9.
A specimen recently placed in the Woodwardian Museum is about the same size as P. Barretti, but possesses very different characters. No interstitial striations are visible ; the ribs are nume- rous, and ornamented with closely set, narrow, lamellated imbrica- tions. It appears to agree more closely with P. Raulinianus than with any other known to me ; indeed, as far as can be seen, in ornamentation it agrees exactly with that species.
Pecten stjbactjtus, D'Orb.
Pecten subacutus, D'Orb. Pal. Fr. p. 603, pi. 435. figs. 5-10.
This species has about 25 simple ribs, ornamented with squamose elevations set at short intervals ; the ribs are rounded on the upper valve, but more angular and acute on the lower ; slight striations run down each side of the ribs into the interspaces, which are nearly as broad as the ribs.
It is a very rare Pecten at Cambridge ; and most of those I have seen were much worn, so that the ribs appeared quite plain ; but a
