DEVELOPMENT OF SOME SILURIAN BRACHIOPODA 351 



ened, i. e. elemental in every respect. In the second stage of 

 growth (after the appearance of plications on the surface, 

 dimensions 1.5 X 1.1 mm.), the apertural margins have become 

 thickened, and directly thereafter the deltidial plates begin 

 to develop, gradually narrowing the aperture at the base. 

 The symphysis of these plates with the valve is marked by 

 distinctly elevated lines. In maturity the deltidial plates 

 have developed sufficiently to close completely the lower 

 part of the aperture, coming together behind the beak of the 

 dorsal valve, and giving to the foramen an elliptical outline 

 constricted toward the apex, where it encroaches upon the 

 umbo. The fact that the development of the foramen is 

 thus interrupted before it reaches the circular outline normal 

 to the adult of most Paleozoic species indicates an embryonic 

 character in the adult, and therefore a subordinate taxo- 

 nomic position for the species. 



Plications. — These appear only after the first stage of 

 growth is passed and after the first growth-line has been 

 formed. As in Homaospira evax, 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 XVIII, figures 12-22.) 



Hall. Twenty-eighth Ann. Kept. N. Y. State Mus. Nat. 



Hist., p. 167, pi. 25, figs. 29-38, 1879. 



Hall. Eleventh Ann. Kept. 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 Oamarotcechia neglecta, Homoeospira evax, Whitfieldella 

 nitida, and others. This seems to be due to the uniformity 

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



