THE TERTIARY FORMATION. 53 



It is abundant in the Chalk of England and Europe, and occurs also in the Upper 

 Greensand, but not in the Gault. I have it also from the Miocene of Virginia, U.S. 

 It has near allies in the Magnesian and Carboniferous Limestones of Britain, and in 

 the Carboniferous Shales and " Bituminous Limestone " of Southern Australia. 



With this extensive distribution of the species in question, there is, of course, much 

 variation in comparatively unessential characters, especially in the superficial papillae 

 (bases of setae) and punctation, and the setation of the surface and extremities ; the 

 relative size of the carapace and its angularity also vary considerably ; and I believe 

 that the lucid spots will be found on careful examination of transparent valves to 

 present some differences of form (a character probably of more importance than any 

 afforded by spines or pittings). 



In the specimens from the Crag of Sutton and elsewhere in Suffolk, the carapace is 

 larger, has a somewhat more rounded outline (fig. 2 a), and is more globose than the 

 Chalk form ; and the lucid spots, though arranged in a similar rosette-like pattern 

 (fig. 2 b), are further apart, and, not being compressed one against the other, have 

 more oval outlines. The surface of the valves, from the presence of papillae, appears 

 to have been setous. The very small individual (fig. 3) occurred in the Red Crag 

 of Walton, Essex ; and one specimen of a narrow variety was met with in the Suffolk 

 Crag. 



The London Clay of Copenhagen Fields, near London, has yielded some handsome 

 specimens (pi. 6. figs. 1, 2), covered with a close punctation, and finely denticulated at 

 the extremities, — conditions not unfrequently met with in recent specimens. 



No. 2. Bairdia contracta, spec. nov. Plate V, figs. 1 a — 1 c. 



INCH. 



Length, Jj Middle Eocene : Barton, Hampshire. 



Carapace elongate-triangular, sub-cylindrical ; most convex at the middle of the 

 ventral portion ; rounded in front ; obliquely acute behind ; sinuate on the ventral, 

 and arched, with an obscure three-sided outline, on the dorsal border; hinge-line 

 occupying the middle third of the dorsal edge : surface smooth. 



Dorsal aspect narrow-acute-ovai ; end-vieio sub-ovate. 



This species approaches Bairdia cylindracea, Bornemann (' Zeitsch. Deutsch. geol. 

 Ges.,' vii, p. 359, t. 20, fig. 5), from the Septarian Clay of Hermsdorf, near Berlin. 



Cytliere {Bairdia) contracta was found by Mr. F. Edwards in the Barton Clay, 

 Hampshire : it is rare. 



