THE TERTIARY FORMATION. 25 



The following localities yield varieties apparently indistinguishable from C. punctata, 

 — Southend (near the mouth of the Thames), Poole Bay (Dorset), Devonshire Coast, 

 Tenby, and Arran ; also Australia. 



Numerous allied forms of this punctated peach-stone-like group of Cytheres, 

 generally of small size, are common in the Oolitic, Cretaceous, and Tertiary deposits, 

 and in the present seas. 



No. 2. Cythere trigonula, spec. nov. Plate III, figs. I, a — \,h. 



INCH. 



Length, ^ Pliocene : Suffolk. 



Carapace ovately subtriangular, depressed on the anterior, dorsal, and posterior 

 borders, and strongly convex ventrally, where the valves are, as it were, pinched up 

 into an obtuse ventral ridge, surmounted by a slight longitudinal fold ; the surface of 

 each valve gradually rises outwards from the dorsal edge to the ventral border, and is 

 then suddenly bent inwards, so that the ventral surface of the closed carapace is flat, 

 and that of the dorsal culminate; the transverse section of the carapace being 

 triangular : surface finely punctate, with small round pits (fig. 1 h) : hinges strongly 

 developed. 



Dorsal aspect lanceolate ; end view triangular. 



Cythere trigonula is plentiful, and accompanies C. punctata, in the Crag of Suffolk. 

 Its triangular shape and larger size well distinguish it from its companion ; and the 

 pits of its punctation are relatively smaller. 



No. 3. Cythere triangularis, Reuss. Plate VII, figs. 5 a — bg. 



Cythere triangularis, Reuss. Zeitsch. Deutsch. Geol. Gesell., vii, p. 279, t. 10, fig. 3. 



INCH. 



Length,-^ Lower Eocene: London. 



Cretaceous : Basdorf, Mecklenburg. 



Carapace gibbose, ovato-triangular ; dorsal margin straight ; extremities obliquely 

 rounded and toothed ; hinder end narrowest ; ventral portion of each valve strongly 

 convex, surmounted by a narrow, well-defined, spinous, longitudinal, slightly curved 

 ridge or fold, and turned suddenly inwards, so as to form a fiat ventral face to the 

 closed carapace : valves thin, smooth, shining, and beset with scattered setae : hinge 



4 



