150 CRETACEOUS LAMELLIBRANCHTA. 



EemarJcs. — The appearance of the shell differs consi(leral)ly according- to the 

 state of preservation ; the fiie concentric grooves on the right valve are seen 

 chiefly when the onterniost layer of the shell lias Ijeen 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, in 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. Trnlhj/ Limestone (zone of Bel. hrunsvicensis). — -The forms found at this 

 horizon never reach a large size (PI. XXVII, figs. 1, 2), and on the average 

 are smaller than those found in the Cenomanian ; they ai'e also slightly higher in 

 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 vunbo 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 Cault (PI. 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. 



4. Upijer Greensand. — The forms from Ventnor are noteworthy for the large 

 size which they reacli ; the number of concentric I'ibs is variable, and on the largest 

 specimens the later part of the valve (PI. XXVII, fig. 7 a) is smooth and without 

 ribs, and in tlie riljl)ed part two distinct stages may sometimes be noticed, an 

 earlier with close-set ribs, a later with more widely separated ribs (PI. XXVII, 

 figs. 8, 9). Sometimes the grooves are widely spaced and the ribs broad (fig. 9 a). 



/'. orhicuhiris seems to lie rare at Blackdown, and tlic forms seen are rather 

 small, with ininierous I'ibs. The example's fi'oiii the Chert Beds of Ilaldon 

 (PI. XXVMl, fig. I), of which there is a good series in the Exeter Miiseun), liavethe 



