PECTEN. 207 



Greensand of Ventnor are ratlier more convex than usual ; those from the Upper 

 Greensand of Warminster are, as a rule, less convex and rather longer. Other 

 modifications are seen in the strength of the main ribs, the flatness or slightly 

 concave character of the interspaces, the number of ribs in the interspaces and 

 the amount of their inequality, and the length of the hinge-line, which is corre- 

 lated with the slope of the anterior and posterior areas. 



In some specimens from the Upper Chalk (Plate XL, fig. 2) the main ribs 

 are rather stronger than usual, the intermediate ribs more unequal, and the hinge- 

 line shorter ; but such forms can be matched with some of those found in the 

 Cenomanian of Devon and France, whilst other Upper Chalk specimens belong to 

 the type which is common in the Upper Greensand of Warminster and the 

 Cenomanian of the Devon coast and France. 



The specimens figured by Mantell were regarded by d'Orbigny as distinct from 

 Pecten quinquecostatus, Sowerby, and were referred by him to Janira Dutemplei. 

 Peron adopts the same view, and moreover refers the first two of Sowerby's 

 figures (figs. 4, 5) to P. Dutemplei, and regards the Upper Greensand form as 

 distinct. An examination of a number of specimens from the Upper Chalk shows, 

 however, that whilst there is some variation, yet they are inseparable from the 

 Upper Greensand form, and that they differ from P. Dutemplei (see p. 216). Morris 

 also appears to have regarded the Warminster specimens as distinct from those 

 found in the Chalk ; the latter he referred to P. quinquecostatus, and the former 

 to P. sequicostatus ; but the Warminster form figured by Sowerby is certainly 

 distinct from P. sequicostatus, Lamarck. A typical form of P. quinquecostatus from 

 the Cenomanian of Rouen is figured by Bayle, and similiar specimens from that and 

 other French localities have been sent me by M. Raoul Fortin of Rouen. In some 

 Cenomanian specimens the main ribs are not so strong as usual ; such forms make 

 some approach to P. sequicoxtatus, but can be easily distinguished by their well- 

 marked concentric ridges. 



Types. — In the British Museum; figs. 4, 5, from the Chalk of Lewes; figs. 

 G — 8 from the Rye Hill Sand of Chute Farm, Warminster. Mantelfs figured 

 specimens appear to have been lost. 



Distribution. — Lower Greensand of Faringdon and Upware. Folkestone Beds 

 of Folkestone. 



Upper Greensand (zone of Schlambachia rostrata) of Blackdown ; malmstone of 

 Devizes and the Isle of Wight. Upper Greensand (zone of Pecten asper) of 

 Haldon, Cheddington (Dorset), Ball Wood (Dorset), Shaftesbury, Warminster, 

 Ventnor, and Niton. 



Rye Hill Sand of Warminster. Chloritic Marl of Maiden Bradley. Cenomanian 

 (Meyer's Beds 10 and 12) of Dunscombe and (Bed 11) of Branscombe and 

 Whitecliff. Cenomanian Sandstone of Wilmington. T. gracilis zone of Dover 



