UPPER, OR FLINTY CHALK. 203 



margin is however perfectly smooth, the hinge hne straight, and the length 

 of the shell exceeds its width, in a greater proportion than in the present 

 species. 



Localities. Upper chalk, near Lewes, and Brighton. 



66. Pecten . Tab. xxv. fig. 14. 



The specimen here figured, is in too anutilated a state to admit of 

 specific distinction ; but as a rare production of the chalk, it is worthy of 

 notice. It is a thin, delicate, shghtly convex shell, of a suborbicular form, 

 marked with gentle undulating grooves, or furrows. The margin is entire, 

 and smooth ; no transverse striae are perceptible, but the Hues of increase 

 are strongly marked. The figure is reduced one-third. 



Locality. Upper chalk, near Lewes. 



67. Pecten . Tab. xxv. fig. 6. 



The cast of a small pecten, its surface marked with eleven radiating 

 costae ; — the imperfect state of this fossil prevents the determination of its 

 species. 



Locality. Upper chalk, near Lewes. 



68. Plagiostoma spinosa. Tab. xxvi. fig. 10. 



Obovate, inequivalve, longitudinally furrowed ; the flatter valve armed 

 with spines : sides nearly equal ; beaks incm-ved, approximate ; ears small, 

 even ; margin denticulated. 



This fossil is remarkably distinguished from the other shells of the 

 chalk, by the long slender spines attached to the upper valve. 



Each valve has from 25 to 30 rounded costae, formed by intervening 

 furrows, that radiate from the beaks to the margin ; these are decus- 

 sated by fine transverse striae, and the Unes of increase ; the inner sur- 

 face of the shell is also marked with corresponding impressions. The 

 lower valve is most convex, and has the line of the hinge straight ; the 

 upper valve is spinous, rather depressed, and contains the angular sinus, 

 by which the shells of this genus are characterized. 



The spines arise from the ribs, but without any regularity, except that 

 they are more numerous at the sides, than in the centre. They vary from 

 15 to 20 in number, and are from half an inch, to two inches and a half 



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