272 



Surface marked with sharp longitudinal ridges. There are 

 two very closely allied forms of this species. In fig. 7 the 

 anterior margin is thickened and pitted, and the ridges, instead 

 of occupying the middle of the valve (as in fig. 8), occupy the 

 posterior half : fig. 7 has also a longer valve, and the dorsal region 

 differs from that of fig. 8, in not being compressed. Hingement, 

 as in pi. L, fig. 2. 



Two specimens of fig, 7 from the Blue Fullers-earth Clay ; 

 and one specimen of fig. 8 from the Blue, and one from the 

 Yellow Fullers-earth Clay of Midford. 



57. CYTHERTDEA CRATICULA,* sp. 71. 



PI. iv., figs. 9 a, h, c, and 10 a, b, c. 



These two specimens appear to us to belong to one form, fig. 9 

 being weaker in its details, and narrower in the posterior region 

 than fig. 10. Taking the latter (fig. 10) as the type, we may 

 describe it as having an oblong carapace, with an angular posterior 

 margin. The surface bears several longitudinal ridges, more or 

 less curved, coarsely reticulate in the interspaces, the meshes 

 passing up on the sides of the largest ridges, making them 

 fenestrate. We consider fig. 9 rather as a weak variety, than as 

 an immature form of fig. 10. Hingement, as in pi. i., fig. 2. 



Of fig. 9, one specimen from the Yellow Fullers-earth Clay ; 

 of fig. 10^ two specimens from the Blue Fullers-earth Clay, 

 Midford. 



58. CYTHERIDEA BRADIANA, JoneS. 



PI. iv., figs. 11 a, I, c. 



Cythere Bradiana, Jones, Quart. Journ. Geol. Soc, vol. xl., 1884, 

 p. 772, pi. xxxiv., figs. 38 a, b. 



* A gridiron 



