PECTEN. 199 



Affinities. — Whilst recognising the close resemblance which exists between 

 /'. atavus and /'. 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. 



Janira valangiensis, Pictet and Campiche, 2 differs from Pecten atavus chiefly in 

 the convexity of the left valve. 



Be marks.— The English examples of this species arc 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. (iV.) atava by W. Keeping, is 

 considerably worn, and the ribs in the interspaces are stronger than usual; 

 consequently I am 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 j but this form also occurs in Brunswick, and Wollemann has shown 

 that it cannot be separated from / ; . atavus. 



Types. — Reiner's types came from the Ilils-conglonierate of Schandelah and 

 Schbppenstedt. 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 /'. neocomiensis came from the Hauterivian near Neuchatel, etc. The 

 specimen figured by Keeping as /". alums is in the collection of Mr. J. F. Walker. 

 The types of /'. omithopus are in the Woodwardian Museum. 



Distribution. — Lower Grreensand of Upware and Faringdon. Ferruginous 

 Sands of Shanklin. 



1 • Foss. Corall. Valang. .■( Crgon. de Mt. Saleve ' (18(36), p. G5, pi. E, fig. !'. Also in A Favre, 

 • Rech. -vol. dans Savoie,' vol. i ( L867), p. 390, pi. C, fig. 27. 



- ' Foss. Terr. C'n't. Sir. Croix ' (1870), p. 242, pi. clxxxi. figs. 1—3. 



