PECTEN. 217 
widely separated—in P. quinquecostatus these ridges are continued from the ribs 
across the grooves without change. See also 1. striato-costutus (below). 
The figure of Pecten sexcostatus given by Woodward is not suffi- 
Remarks. 
ciently good for exact determination, but specimens which have been obtained 
from the same locality as the type leave no doubt as to the identity of P. sexcos- 
tutus with Jamra Dutemplei, D’Orbigny. 
The examples from the Lower Chalk are regarded by Peron as specifically 
inseparable from those found in the Upper Chalk, and with this view I agree, but 
since some slight differences are generally noticeable, I think it may be well to look 
on the examples from the Lower and Upper Chalk respectively as two forms or 
varieties of the same species, which may be referred to as Form a and Form B.A 
careful comparison of good series of specimens can scarcely leave any doubt on the 
mind as to the later forms having descended directly from the earlier. The 
examples found in the Gault probably also belong to Form a; they have, however, 
usually been referred to P. (Neithea) quinquecostatus. 
The strength of the main ribs and the convexity of the right valve vary 
considerably in different specimens. ‘There are also variations in the number of 
intermediate ribs, in their relative sizes, and in the depth of the incisions of the 
margin between the main ribs. In some examples from East Harnham the main 
ribs are unusually strong. In some of the larger specimens from Norwich the 
number of ribs near the margin of the valve is greater than usual owing to the 
introduction of small ribs in the grooves. 
T'ypes.—W oodward’s type appears to have been lost. D’Orbigny’s type of 
Janiva Dutemplei came from the Senonian of Chavot (Marne). 
Distribution —Form a.—Chalk Marl of Dover, Folkestone, and Ventnor. 
Totternhoe Stone of Burwell. H. subglobosus zone of Cherry Hinton. 
Form B.—A. quadratus zone of Kast Harnham (Salisbury) and Winchester. 
B. mucronata zone of Ballard Head (Dorset), Clarendon (Salisbury), and Norwich. 
Upper Chalk (? zone) of Brighton. Recorded by Rowe from the M. cortestu- 
dinarium and M. cor-anguinum zones of Thanet. 
Proven (Nerruea) striavrocostatus, Goldfuss,' 1833. Plate XLI, figs. 9, 10. 
A portion of a right valve and a nearly perfect left valve, which were found by 
1 «Petref. Germ.,’ vol. ii (1833), p. 55, pl. xeili, figs. 2 ¢, d, e (not 2 a, b, f, g) ; Favre, ‘ Moll. Foss. 
Craic de Lemberg’ (1869), p. 156, pl. xiii, figs. 12, 13 ; Holzapfel, ‘ Moll. Aachen. Kreide’ (1889), p. 239, 
pl. xxvi, fig. 19; Vogel, ‘Holliindisch. Kreide’ (1895), p. 26; Miiller, ‘Mollusk. Untersen. v. Braunschweig 
u. Ilsede’ (1898), p. 37; Ravn, ‘Mollusk. i Danmarks Kridtaflej.’ (1902), p. 95, pl. ii, figs. 8, 9; Wolle- 
mann, ‘ Mauna der Lineburg. Kreide’ (1902), p. 63. 
