DEVELOPMENT OF SOME SILURIAN BRACHIOPODA 335 



Niagara Formation at Waldron, Indiana."* Fortunately, 

 there has been more recently detected a young individual of 

 about one-fourth the normal adult size, which offers some 

 interesting details in its form and characters. 



The asymmetry of the shell is manifest even at this early 

 stage of growth (although the median fold is not developed), 

 and is evinced by the position of the beak of the ventral 

 valve and by the contour of the margins. It is probable 

 that in a still earlier phase of growth the two valves are 

 symmetrical, or nearly so. 



In the young individual under consideration (Plate XA^II, 

 figures 9, 9 a, 9 5), the outline is nearly circular. The beak 

 of the ventral valve is very much elevated, projects beyond 

 the cardinal line, and is directed toward the left side of the 

 shell. The apex is truncated, and the opening is confluent 

 with the area below. 



The cardinal area is high, forming a large triangular fissure 

 which is apparently not closed by deltidial plates. The beak 

 of the dorsal valve is depressed, and limited by a slight fur- 

 row on each side. No lines of growth are visible, but the 

 surface is somewhat granulose, as in many small shells of 

 other species. 



The principal differences to be noted in comparison with 

 the adult individuals are the sub-circular outline of the shell, 

 the depressed valves, the absence of a median fold, and the 

 large deltidial area. 



Bictyonella reticulata Hall, 1868. 

 (Plate XVII, figures 11-13.) 



Eichivaldia reticulata Hall. Twenty-eighth Ann. Rept. N. Y. State Mus. 



Nat. Hist., p. 169, pi. 26, figs. 50-54, 1879. 

 Hall. Eleventh Ann. Rept. State Geol. Indiana, p. 312, pi. 26, 



figs. 50-54, 1882. 



Very few of the earlier growth-stages of this species have 

 been observed, and these show but comparatively little varia- 



* James Hall. Read before the Albany Institute, March 18, 1879. 



