386 STUDIES IN EVOLUTION 



exact bomologue of the deltidial plates in Terebratula, Rhyn- 

 chonella, etc., and to be radically different from the deltidium 

 of Strophomena, Stropheodonta, Orthothetes, and allied genera. 



Summary of Developmental Changes. 



Size and Contour. — Although the species described in the 

 preceding pages present a wide variation in form and general 

 appearance, the nature of the changes which take place in the 

 development of the shell is remarkable in its uniformity. 



In nearly every species the inceptive state is represented 

 by a shell having a sub-circular outline, with valves of slight 

 convexity. This phase usually disappears before the indi- 

 vidual reaches a length of 1 mm., after which the specific 

 characters are assumed, and are progressively emphasized 

 with each succeeding increment. 



On comparing the incipient stage in these fossil shells 

 with that of recent brachiopods, as given by Mr. E. S. Morse 

 for TerebratuHna and by Mr. W. K. Brooks for Glottidia, 

 it is found that, in respect to actual size, there is a slight, 

 though perhaps unessential difference. At the earliest stage 

 of growth figured by Morse,* the shell has a length of about 

 .3 nun. and in the next stage represented, of approximately 

 .6 mm. 



The first two stages of the shell figured by Mr. Brooks f 

 represent free animals, and measure .24 and .3 mm. in 

 length, respectively. The shell became attached by the 

 pedicle only upon attaining a length of 2.5 ram. 



Most of the fossil forms have furnished evidence, either 

 from actual elemental specimens or from the apical portions 

 of subsequent incipient stages, that the true initial shell did 

 not reach a size of more than .5 mm. in length. Soon after 



* On the Early Statues of Terehratxlinn septentrionalis. Mem. Boston Soc. Nit. 

 Hist., II, pi. i. figs. 2, 3, 1869. 



t The Development of Linr/nln and the Systematic Position of the Brachiopoda. 

 Johns Iloiikins Universitij, Chesapeake Zodl. Lab., pis. i and ii, 1879. 



