THE TERTIARY FORMATION. 45 



Cytheridea Jonesiana, Bosquet. Mem. Couron. Acad. Belg., xxiv, p. 38 ; and Mem. 



Comm. Carte G6ol. Neerl., ii, p. 64, t. 8, fig. o. 

 — — Reuss. Denksch. Akad. Wiss. Wien., vii, p. 141. 



Bairdia perforata, Bosquet. Mem. Couron. Acad. Belg., xxiv, p. 24, t. 1, fig. 8. 



INCH. 



Length, ^3. Tertiary: England; Europe. 



Cretaceous : England ; Europe. 



Carapace sub-triangular or triangular-ovate ; most convex at or just behind the 

 centre of the ventral portion ; ventral border straight, or faintly convex ; dorsal border 

 strongly arched and somewhat angular at the anterior hinge ; anterior end broad and 

 obliquely rounded ; posterior tapering and obtuse ; right valve much narrower than the 

 left, its hinge-line oblique and terminating at the anterior hinge with a distinct angle ; 

 surface of the valves finely punctate and perforate, the perforations, obliquely seen in 

 the translucent substance of the carapace, often appearing as projecting spines or 

 hairs (see Note, p. 44). 



Dorsal aspect more or less lanceolate ; end-view nearly ovate. 



The series of Tertiary and Cretaceous Entomostraca, from France and Belgium, 

 with which M. Bosquet has kindly favoured me, comprises a specimen of C. perforata, 

 Roemer ; and hence I am enabled to point out that there are no essential differences 

 between the forms enumerated above in the list of the Synonyms of this species. The 

 greater relative size and ovateness of the left valve, — the apparent presence of setse, 

 arising from optical appearances under the microscope, in some specimens, — slight 

 variations in the convexity of the surface and the coarseness of the punctations, and 

 the occasional presence of marginal spines— constitute the differences among specimens 

 from different localities. 



Cytheridea incrassata, Bosquet (' Entom. Tert.,' p. 44, t. 3, f. 11), is evidently a 

 closely allied form ; and so also are Bairdia sub-trigona, Bornemann ('Zeitsch. Deut. 

 geol. Ges.,' vii, p. 357, t. 20, f. 4), and Cytheridea punciatella, Bornemann (ibid., p. 360, 

 t. 21, f. 2). 



Cy there {Cytheridea) perforata occurs in the Upper Eocene Clays at Barton, on 

 the Hampshire coast, and in the sands of the same age at Colwell Bay, Isle of Wight; 

 (and two varieties in the London Clay, near London). M. Bosquet found it 

 (B '. perforata) in the " Sables moyens," Tancrou (Seine-et-Marne), and the " Calcaire 

 grossier," Damery (Marne) and Montmirail (Aisne). Roemer derived his specimens 

 from the Paris Tertiaries. I have it also from the " Calcaire grossier superieur." 



As a Cretaceous form, it occurs in the Maestricht Chalk at Kunraede (Bosquet), in 

 the Chalk of Balsberg, in Sweden, and of Kent (rare), in the Chalk-marl of Charing 

 and Dover, in the Gault of Kent, and the Greensand (so-called) of Blackdown. 



