PECTEN. 207 
Greensand of Ventnor are rather 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. xquicostatus ; but the Warminster form figured by Sowerby is certainly 
distinct from P. equicostatus, 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. equicostatus, 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. 
6—8 from the Rye Hill Sand of Chute Farm, Warminster. Mantell’s figured 
specimens appear to have been lost. 
Distribution—Lower Greensand of Farmgdon and Upware. Folkestone Beds 
of Folkestone. 
Upper Greensand (zone of Schlanbachia 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. 7’. gracilis zone of Dover 
