THE TERTIARY FORMATION. 51 



Cythere (Cytherideis) Jlavida 1 abounds in the recent state along the coasts of Britain, 2 

 and in the Baltic. It is abundant in the Crag of Suffolk ; and occurs also, but much 

 less frequently, in the Upper Eocene Lands of Colwell Bay, Isle of Wight. 



No. 10. Cytherideis Ren, spec. nov. Plate IV, figs. 5 a, 5 b. 



INCH. 



Length, 2 V Pliocene: Suffolk. 



Carapace quadrangularly reniform ; extremities obliquely rounded, curving up to 

 meet the short straight hinge-line occupying the central third of the dorsal border ; 

 anterior extremity smaller and more oblique than the posterior ; ventral border sinuous, 

 strongly incurved at the middle : surface rather depressed, smooth, with the exception 

 of some obscure papillae and pittings, partially distributed. Lucid spots eight, long- 

 oval, arranged in two oblique transverse rows in the middle of the valve and rather 

 ventrally. 



Dorsal aspect narrow-oblong, slightly tapering, and with rounded ends ; end-view 

 sub-oval. 



Cythere {Cytherideis) Ren is perhaps related to C. pilosella, Reuss. 



This species is from the Crag of Suffolk, and is rare. 



Sub-yenus — Bairdia, 3 31' Coy, yen. 



Animal a Cythere. Carapace varying from a broadly triangular to a narrow elon- 

 gate sub-triangular form, with extremities more or less acute ; surface smooth and 

 setiferous or finely punctate ; no central tubercle ; lucid spots well marked : maryins 

 thin and trenchant ; when closed, the edges of the right valve lie within those of the 

 left : interior of the marginal borders (except on the dorsal edge) cased with a narrow 

 lamelliform plate (as in Cypris) : except that a slight fold or notch is frequently appa- 

 rent at the angles of the hinge-line, the dorsal edge of the right valve is quite simple, 

 and, in the closed carapace, underlies the dorsal edge of the left valve, which is larger 

 and overlapping : ventral margin incurved. 



Among the species which I assigned to Bairdia, in 1849, some appear to me to 

 be sufficiently distinct, especially in their hingement, to be divided off and arranged 



1 Zenker has well remarked that colour is a bad characteristic for specific determinations of these 

 Entomostraca ; the colour often varying with different food and other circumstances. With regard to the 

 species referred by Zenker to C.Jlavida, Midler, I have already remarked (see p. 23). 



2 I have procured it from Southend, Margate, Poole Bay, Tenby, and Scarborough ; and Dr. Baird 

 quotes it from Torquay and Berwickshire. 



3 See 'Monog. Entom. Cret.,' 1845), p. 22. 



