THE TERTIARY FORMATION. 37 



convex posteriorly and ventrally ; sloping suddenly towards the borders, slope steepest 

 on the ventral border: surface ornamented with a large central tubercle and two thin' 

 longitudinal, parallel, beaded ridges ; one of which is placed at the edge of the dorsal, 

 the other at that of the ventral slope ; and both are connected by a similar, short, 

 transverse ridge along the edge of the posterior slope ; the area between and outside 

 the ridges is faintly punctate. 



Dorsal aspect elongate-acute-ovate, nearly cuneiform ; end-view sub-ovate. 



I have met with only three specimens in the Crag of Suffolk. 



Sub-genus — Cythereis, 1 Jones. 



Animal probably a Cytliere. Carapace oblong ; variously ornamented with reticu- 

 lations, tubercles, spines, and ridges : the bar and furrow of the hinge are nearly or 

 quite obsolete ; but the anterior and posterior hinges are well defined, and isolated, 

 marking definite angles in the outline of the carapace-valves : the central and posterior 

 tubercles on the surface of each valve are strongly marked, passing into more or less 

 developed longitudinal ridges, the ventral one of which is always raised, and often 

 greatly produced, giving a flat ventral surface and a somewhat triangular end-view to 

 the closed carapace. 



No. 1. Cythereis senilis, spec. nov. Plate III, figs. 8 a, 8 b. 



INCH. 



Length, -Jg Pliocene : Suffolk. 



Carapace-valve oblong, rounded in front, truncate and denticulate behind ; 

 margins thickened ; surface bearing three disconnected, squared ridges ; the one 

 towards the ventral border running the length of the raised plateau of the surface and 

 curving round its anterior portion ; the central ridge shorter and nearly straight ; the 

 dorsal one shortest and oblique ; exposed edge of the ridges beaded, or rather marked 

 with lozenge-shaped spots, which possibly indicate that the ridges were once higher 

 and perforate, as in Cythereis fistulosa, C. runcinata, and C prava, Baird, — the lozenge- 

 markings being perhaps the bases of the minute interstitial pillars of the raised perfo- 

 rate edges of the ridges. 



1 For synonyms and greater detail of description, see 'Monog. Entom. Cret.,' 1849, p. 14. Since 

 1849, Cythereis has been in some cases erected into a genus, in others merged into Cythere proper, and 

 rarely accepted as a sub-genus. See Note above, p. /. 



