tirviov 0*6tjj. 57 



Fig. 6 a. View of ventral valve, showing the beak of the dorsal valve. 



Fig. 6 b. View of the dorsal valve. 



Fig. 6 c Profile view of the shell. 



Fig. 6 d. Front view of the same. 



Fig. 6 e. Cardinal view of the same. 



Fig. 6 f. Cast of the dorsal valve of this species. 



Fig. 6 g. The striae enlarged, showing their mode of bifurcation and increase. 



Position and locality. This species occurs in the lower shaly calcareous beds at Reynale's 

 basin, Niagara county. 



440. 22. ORTHIS ELEGANTULA 1 Var. 



PL. XX. Fig. 7o-fc. 



Compare Orthis elfganlula, Dalman. 



For references and synonyms, see 0. elcgantula under Niagara group in this volume. 



Dorsal valve circular, being as wide as long, very convex and much elevated towards the 

 beak ; striae not much elevated, of equal size and evenly bifurcating ; cast of the muscular 

 impression oblong, pointed above, and having a distinct impressed line towards the lower part. 



Several of the specimens figured have many features unlike the ordinary forms of 0. 

 eleganfula., but a careful examination has shown no reliable characters for specific distinction. 

 Many of the specimens are larger than any individuals of that species in the Niagara group, 

 where it reaches its maximum of development in regard to numbers. The dorsal valve is more 

 often round, and it is usually circular or nearly so, having a proportionately greater width 

 than specimens in the Niagara group. This greater width, however, may arise in part from 

 pressure, as the specimens are less elevated than those of the Niagara group which have not 

 suffered pressure. 



Fig. 7 a. Dorsal valve of a specimen of medium size, somewhat distorted by pressure. 



Fig. 7 b. Hinge-line and beak of dorsal valve. The hinge-line of the ventral valve is not quite 



so straight as usual in O. elegantula. 

 Fig. 7 c. Dorsal valve of a large individual. 

 Fig. 7 d. Profile view of 7 c. 



Fig. 7 e. The cast of a dorsal valve of a small individual. 

 Fig. 7/. Cast of the dorsal valve somewhat compressed. 

 Fig. 7 g. Dorsal valve with the shell partially removed. 

 Fig. 7 h. Dorsal area of 7 g. 

 Fig. 7 i. Showing the flattened striae. 

 Fig. 7 k. Several of the striae enlarged. 



The forms here figured, which are some of the best defined, would not be readily recognized 

 by the unaided student as belonging to the species, and it therefore seems desirable to illustrate 

 it under these phases. 



[ Pal^ontoloct — Vol. 11.] 8 



