PECTEN. 199 
Affinities —Whilst recognising the close resemblance which exists between 
P. atavus and P. neocomiensis, Pictet and Campiche thought that the latter could 
be distinguished from the former by the absence of ribs in the interspaces. 
Wollemann, however, by means of a much larger series of specimens, has shown 
(1896) that the differences which were supposed to separate the two forms are 
not constant. 
The form described by de Loriol' as Janira Matheroniana is very similar to 
P. atavus, and may prove to be identical with it. 
Janirva valangiensis, Pictet and Campiche,’ differs from Pecten atavus chiefly im 
the convexity of the left valve. 
Remarks—The English examples of this species are of rather small size, the 
average height being about 26 mm.; none of them reaches the size of the large forms 
figured by d’Orbigny and by Pictet and Campiche. Many of our examples are 
more or less inequilateral, but even in the large specimens from Ste. Croix this 
character is sometimes noticeable. The anterior ear is also distinctly larger than 
the posterior in most specimens. 
The large example from Upware, figured as P. (N.) atava by W. Keeping, is 
considerably worn, and the ribs in the interspaces are stronger than usual ; 
consequently Iam unable to regard it as belonging to P. atavus. It is probably 
only a worn specimen of P. (N.) quinquecostatus. 
On account of their inequilateral character the specimens from Upware were 
regarded by W. Keeping as belonging to a distinct species, which he named 
P. ornithopus ; but this form also occurs in Brunswick, and Wollemann has shown 
that it cannot be separated from P. atavus. 
Types.—Romer’s types came from the Hils-conglomerate of Schandelah and 
Schéppenstedt. Romer’s figure is not quite satisfactory, but any doubt which may 
formerly have existed as to the nature of his species has been removed by 
Wollemann, who has obtained other specimens from the same locality. D’Orbigny’s 
specimens of P. neocomiensis came from the Hauterivian near Neuchatel, ete. The 
specimen figured by Keeping as P. atavis is in the collection of Mr. J. F. Walker. 
The types of P. ornithopus are in the Woodwardian Museum. 
Distribution—Lower Greensand of Upware and Faringdon. Ferruginous 
Sands of Shanklin. 
1 «Foss. Corall. Valang. et Urgon. de Mt. Saléve’ (1866), p. 65, pl. E, fig. 9. Also in A. Favre, 
‘Rech. géol. dans Savoie,’ vol. 1 (1867), p. 390, pl. C, fig. 27. 
2 «Boss. Terr. Crét. Ste. Croix’ (1870), p. 242, pl. clxxxi, figs. 1—3. 
