SILURIAN BRACHIOPODA 17 



change their mutual angle, constantly lessening it as growth ad- 

 vances. 



Plications. As noticed above, the earliest stages of growth observed 

 show the striae to be already developed on the shell, five on the ventral 

 and six on the dorsal valve. These plications are rapidly multiplied by 

 interstitial addition, and in maturity number from one hundred to one 

 hundred and thirty on each valve. 



Orthis hybrida, Sowerby, 1839 



Plate I, Figs. 13-18 



, Hall. Twenty-eighth Ann. Rept. N. Y. State Mus. Nat. Hist., p. 149, pi. 21, figs. 



18-25. '879. 

 , Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 285, pi. 21, figs. 18-25. 1882. 



Orthis hybrida passes through primary developmental stages which 

 are essentially identical with those already described for O. elegantula. 

 Sufficient has been said in that connection in regard to the similarity and 

 probable identity of the earlier embryonic stages of the shell of both spe- 

 cies, the origin of the entire specific difference which is so apparent in the 

 later and mature periods of development lying in the unequal growth of the 

 valves in convexity. This increase is relatively greater in the dorsal 

 valve of O. hybrida than in that of O. elegantula, and less in the ventral 

 valve of the former than in that of the latter species. Thus O. hybrida 

 is a more discoid, lenticular shell, showing but slight evidence of a median 

 fold and sinus and carrying on its surface at maturity just about as many 

 plications or striae as its associate. 



There is an obese variation from the normal form of O. hybrida, 

 which was noticed by Professor H.\ll {Joe. eit."), and this appears early in 

 the development of the species, with a size of 3.5 mm. in length and 4.5 mm. 

 in width, and reaches a maximum growth with dimensions of 14x13 mm. 

 This variation is due to internal thickening and increase in convexity, 

 and is accompanied by abundant concentric growth-lines which are as 

 3 



