SILURIAN BRACHIOPODA 47 



they appear over the entire surface of the shell below the growth-line all at 

 once, and from this stage onward to maturity, no increase is made in the 

 number, except by intercalation along the margin of the fold and sinus. 



RHYNCHOTRETA CUNEATA, Dalman, 1827 

 var. AMERICANA, Hall, 1879 



PLATE IV, FIGS. 12-22 



, Hall. Twenty-eighth Ann. Kept. N. Y. State Mus. Nat. Hist., p. 167, pi. 25, figs. 

 29-38. 1879. 



, Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 310, pi. 25, figs. 29-38. 1882. 



The individuals of this species do not so readily separate into three 

 groups of long, normal, and broad forms, as do those of Rhynchonella neg- 

 Iccta, Retzia evax, Meristina nitida, and others. This seems to be due to the 

 uniformity in the number of plications, and also in the number carried on 

 the fold and sinus. The long and broad varieties do exist, however, but are 

 of such infrequency as to suggest that they are not genetic variations from 

 the typical form occurring in this locality. 



The specimens from the Wenlock shales of Dudley, show a consider- 

 able variation from their American congeners, in having more numerous 

 plications, of which a greater number is raised on the dorsal fold and 

 depressed in the ventral sinus. In other respects, it is believed that the 

 description here given, of the development of the shell, will apply to the 

 British form. 



Rhynchotrcta cnncata, although considered as abundant in the mature 

 state, does not approach in the number of young specimens, Rhynchonella 

 whitii, R. ncglecta, R. indiancnsis, Retzia evax, Meristina nitida, Spirifer 

 crispus, Sp. crispus, var simplex, A try pa reticularis, Or this hybrida, and 

 O. elegantula. The entire number of young individuals examined is about 

 one hundred and fifty, ranging in size from 8 mm. to 1.5 mm. in length. 

 The mature forms average about 17 mm. in length. 



