PECTEN. 



155 



occur on the samo geolo^'ical liori/iOns, and since each shows some variation in the 

 proportions of Icngf/li and height, there seems no reason to regard them as more 

 than varieties of one species. 



Tlie concentric grooves are generally more widely se})arated in the specimens 

 from the Sjieeton Clay than in those which come from the Claxby Ironstone. 



'rhis species has nsually been known on the Continent as P. crassitesta, owing, 

 no doubt, to the fact that the type of P. ciiictas came from the Drift, and its true 

 horizon was for some time thono-ht to be Middle Jurassic : also to the fact of its 



Fid. 2.—Pcctcii {Camptonectcs) cinctus. Sow. Claxhy Ironstone, Claxby. Woodwardian Musuuin. 

 The liiylier and shorter form of the species. x 5. 



being the form with a higher shell, which is rather less common than the otiier 

 variety. 



Ti/pe. — In the British Museum ; from the Drift— probably derived from the 

 Claxby Ironstone. 



Distribution. — Claxby Ironstone (zone of Bd. hiferalis) of Claxby, North 

 Willingham, Tealby, Donnington. Tealby Limestone (zone of Bd. l>runsvicensis) 

 of Claxby. Speeton Series (zone of Bel. jacida\ii, D 1, D 4, D 5) of Speeton. 



