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PLATE 32 A. 



Pig. 1. 168. 15. , Okthis subquadkata. (Pag- 126.) 



1 a, b, c. Three views of the dursal valve, showing gradation in the size ol the shell. ^^ 



1 d,e. Profile views, showing the nearly equal elevation of the beaks. '7^ 



1 f, g. Front views, showing the slight elevation produced by the sinus in the ve.ntral valve. Jjf 



1 h, i. Ventral valves, showing the slight depression along the centre of the shell. 



1 k. Cardinal view, showing the area, foramen, etc. 



I /. Exterior of a large dorsal valve somewhat flattened from compression. • 



1 m. Interior of the same, showing the form of the visceral imj>re8sion and the marks of the external radii, 



which terminate near the margin. 

 1 n. Dorsal area and foramen. 



1 o. Interior of the ventral valve, showing the narrow area and foramen, with the projecting medial tooth, 



which is enlarged in the upper figure. The impressions of the external radii are visible but a short 

 distance from the margin on the inside of the shell. * 



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 Fig. 2. 169. 16. Orthis Occident alis. (P^g- 127.) 



2 a -f, are illustrations of a Series of this species, beginning with the smallest recognized specimens, and 



passing through the several grades, till the increasing rotundity of the ventral valve towards the umbo 



rises above the beak of the dorsal valve. 

 2 /f, A. Profile views of several specimens as above. 

 2 t, k, I, m. Front views, showing the increasing sinus of the dorsal valve as the shell becomes older. 



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