DEVELOPMENT OF SOME SILURIAN BRACHIOPODA 317 



usual method employed in tracing the life-history of organ- 

 isms. In this case fossil organisms are to be dealt with, and, 

 in order to insure accuracy of results, it is necessary to begin 

 with the known and established facts and gradually descend 

 to minute and strange forms, thereby connecting the extremes 

 of growth. Under the caption "Developmental Changes," 

 however, an endeavor has been made to trace the history of 

 each feature of the shell, from its inception to maturity. 



DISCUSSIONS OF THE SPECIES. 



Crania siluriana Hall, 1863. 



(PLATE XV, figures 1, 2.) 



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



p. 148, pi. 21, figs. 3-7, 1879. 

 Hall. Eleventh Ann. Kept. State Geol. Indiana, p. 282, pi. 21, 



figs. 3-7, 1882. 



But a single embryo of this species has been found in all 

 the material examined. It is of an incipient stage of growth, 

 measuring but 1 mm. in height, and 1.5 mm. across the 

 aperture. Compared with the mature form, the average size 

 of which is about 9 x 20 mm., it shows a relatively greater 

 elevation and a more regularly conical form. Otherwise all 

 the few essential characters of the adult shell are present at 

 this early age. 



Dalmanella elegantula Dalman, 1827. 



(PLATE XV, figures 3-11.) 



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



Hist., p. 150, pi. 21, figs. 11-17, 1879. 

 Hall. Eleventh Ann. Kept. State Geol. Indiana, p. 285, pi. 21, 



figs. 11-17, 1882. 



Both species of Orthidce occurring at Waldron (Dalma- 

 nella elegantula and Rhipidomella hybridd) are very abun- 

 dant. In the later stages of growth the former species is 



