DEVELOPMENT OF SOME SILURIAN DllACHIOPODA 339 



although in the elemental shell the difference is scarcely per- 

 ceptible, while in the mature form it is a conspicuous feature. 

 This valve is also shorter than the ventral in specimens up 

 to a length of about 7 mm. From 7 to 12 mm., both valves 

 are of nearly equal length. Further growth causes the umbo 

 of the dorsal valve to protrude bej^ond the beak of the oppo- 

 site valve, and the beak is incurved and penetrates the area. 

 It seems evident that if the true initial shell were studied 

 the dorsal valve would be found not only smaller but less 

 convex than the opposite valve. 



The fold begins to be apparent in individuals having a 

 length of about 10 mm., and is expressed by the arching of 

 the anterior margin. It does not sufficiently develop to 

 become a characteristic feature, and is more or less undefined, 

 even in many full-grown specimens. 



The plications increase both by bifurcation and interstitial 

 addition. The smallest number observed is eight, and this 

 is gradually increased with the growth of the shell, until 

 there are about fifteen principal plications on the body of 

 the shell, and several smaller ones just below the cardinal 

 extremities. The concentric striie are not often preserved, 

 and the plications therefore form the only conspicuous char- 

 acter of the surface ornamentation. 



The delthyrium or triangular opening becomes completely 

 filled by the incurved beak of the dorsal valve. 



Camarotoechia acinus Hall, 1863. 

 (Plate XVIII, figures 9-11.) 



Rhynchonella acimis Hall. Twenty-eighth Ann. Kept. N. Y. State Mus. 



Xat. Hist., p. .306, pi. 26, figs. 7-11, 1879. 

 Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 306, pi. 26, 



figs. 7-11, 1882. 



Were Camarotoechia acinus a rare species, it might readily 

 be confounded with the variety of C. indianensis which bears 

 but a single plication in the ventral sinus. It appears, how- 

 ever, to have been very prolific, and its abundance serves to 

 emphasize its specific independence. The liability to con- 



