PLATii 32 A. 



Fig. 1. 168. 15. . Orthis subquadrata. (Pag. IHG.) 



I «, 0, c. 'I'liice views of the dorsal valve, showing gradiition in the size ul the shell. 



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



1 /, g. Front views, showing the slight elevation produced by the sinus in the ventral valve. 



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



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



1 /. Exterior of a large dorsal valve somewhat llattencd from compression. 



1 m. Interior of the same, showing the form of the visceral impression and the marks of tlie external radii, 



which terminate near the margin. 

 1 n. Dorsal area and foramen. 



1 0. 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 insidi- of the shell. 



Fig. 2. 169. 16. Orthis occidentalis. (Pag- 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 g, h. Profile views of several specimens as above. 

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



