150 CRETACEOUS LAMELLIBRANCHIA. 
Remarks.—The appearance of the shell differs considerably according to the 
state of preservation; the fine concentric grooves on the might valve are seen 
chiefly when the outermost layer of the shell has been removed; when a thicker 
layer has disappeared this valve may become almost smooth. 
This is one of the commonest and most widely distributed of the Cretaceous 
Pectens; it varies considerably in size, m proportions, in the number of con- 
centric ribs, and also to some extent in convexity. Some of the varieties are more 
abundant at certain horizons than elsewhere, but are seldom, if ever, limited to one 
level, and are doubtless accounted for by the varying conditions under which the 
deposits were laid down. Some of these varieties are here briefly noticed. 
1. Tealby Limestone (zone of Bel. brunsvicensis).—The forms found at this 
horizon never reach a large size (Pl. XXVII, figs. 1, 2), and on the average 
are smaller than those found in the Cenomanian; they are also shghtly higher im 
proportion to their length, and often rather more convex. The number of con- 
centric ribs is not so great as in many Upper Greensand and Cenomanian forms. 
Near the umbo there is usually seen a fairly large, smooth portion of the shell 
without ribs, which at first sight appears to be a distinguishing feature of the 
forms from this horizon. But the examination of a large series of specimens 
shows that this is due to imperfect preservation ; the same feature has been noticed 
by Dr. Wollemann in Brunswick specimens. The size of this smooth area varies 
very considerably in different specimens, and occasionally nearly all the concentric 
ribs have disappeared; moreover, an identical smooth portion is Sometimes seen in 
specimens from the Gault and Chalk Marl. 
2. Folkestone Beds—Forms very similar to those from the Tealby Limestone 
and of about the same size occur in the Folkestone Beds of Folkestone, but do not 
appear to be numerous. The number of ribs is sometimes greater, sometimes less 
than in the Tealby specimens. 
3. Gault——The forms in the Gault (Pl. XXVII, fig. 3) are, on the average, 
of about the same size as the Tealby specimens, but some larger examples also 
occur; they sometimes possess rather more numerous ribs, and in some cases 
the valves are quite as convex as in the Tealby forms, but usually rather less. 
{. Upper Greensand.—The forms from Ventnor are noteworthy for the large 
size which they reach; the number of concentric ribs is variable, and on the largest 
specimens the later part of the valve (Pl. XXVII, fig. 77) is smooth and without 
ribs, and in the ribbed part two distinct stages may sometimes be noticed, an 
earlier with close-set ribs, a later with more widely separated ribs (Pl. XX VII, 
figs. 8,9). Sometimes the grooves are widely spaced and the ribs broad (fig. 9a). 
DP. orbicularis seems to be rare at Blackdown, and the forms seen are rather 
small, with numerous ribs. The examples from the Chert Beds of Haldon 
(PL XXVII, fig. 4), of which there is a good series in the Hxeter Museum, have the 
