DESCRIPTION OF 



PLATE II. 



Pasje 



Fig. 1. ORTHIS SUBORBICULAEIS. 486 



1 a, b. Dorsal and ventral views of an entire individual. 



1 c. Cardinal view of the same enlarged. 



1 d. Interior of a dorsal valve, showing the brachial and cardinal processes, 



1 e. Interior of a ventral valve of a small individual, showing the cardinal 



teeth. 



Fig. 2 & 3. ORTHIS VANDXEMI. 487 



2 a, b, c. Ventral, dorsal and profile views of a specimen of the usual form 



of the species in its more northwestern localities. 



2 d. Cardinal view, enlarged, to show area and foramen. 



2 e,/. Dorsal and ventral views of a specimen from the same geological 



position in Missouri. 

 2 g, h. Interior of ventral and dorsal valves. 



2 i. Enlargement of strife. 







3 a, b. Ventral and profile views of a specimen of medium size, from the 



shales of the Hamilton group in New- York. 



3 c, d. Interior of ventral and dorsal valves from New-York specimens; the 

 ventral valve showing some slight differences in form and divergence 

 of the teeth from 2 g. 



3 e. Interior of a large ventral valve from the Hamilton group of New- 



York, showing the vascular and muscular impressions : the teeth are 

 broken. 



3/. Enlargement of the striae, showing the punctate surface and more 

 distinct tubular openings. 



Fig. 4. ORTHIS IOWENSIS. 488 







4 a, b. Ventral and dorsal views of a specimen of this species. 

 4 c. Front view of the same. 



4 d. Profile view of specimen. 



4 e,f. Cast of the ventral valve of a young and of a full-grown individual. 



4 g. Cast of the dorsal valve of 4/. 



4 h. Interior of dorsal valve, showing the brachial and cardinal processes. 



4 i. Interior of ventral valve, showing the teeth and the form of the mus- 

 cular impressions, with indistinct radiating lines below, indicating the 

 course of the vascular impressions. 



Fig. 5. ORTHIS IOWENSIS, var. FURNARIUS. 489 



5. Cast of the dorsal valve. 



Fig. 6. ORTHIS INEQUALIS. 490 



6 a. Dorsal valve. 

 6 b. Ventral valve. 

 6 c. Profile view. 



