258 



GEOLOGICAL BIOLOGY. 



records, the greater time-duration covered by its differentia- 

 tion, and because living forms have been carefully examined 

 and their structure and course of embryological development 

 are well known. 



Majority of Characters of Living Brachiopods traceable to Cam- 

 brian Ancestors. — From this tabulation of the range of the 

 Brachiopoda it is evident that a great 

 majority of the characters which any 

 individual Brachiopod exhibits (as a 

 specimen of the Tcrebratnlina scpten- 

 trionalis, now living in large numbers p 



Fig. 6o. 



Fig. s9- 



FiG. 6i. 



Fig. ^c).^Terehrat7dina scptentrionalis. View of the internal structure, the pedicle valve being 

 removed (x 2). pe — pedicle ; r»i = retractor muscle ; .s = shell of brachial valve ; tn = 

 mantle ; am = adductor muscle ; / = intestine ; / = liver lobes ; /p = lophophore ; ne = ne- 

 phridium ; 07/ — ovary. In this figure the pedicle end is the lower. 



FiGS. 60, 61. — Shell of a Terebratula. ,^^5 = antero-posterior axis; CD = horizontal axis; 

 V = ventral or pedicle valve ; D = dorsal or brachial valve ; / = pedicle ; y" = foramen ; 

 c = cardinal slope ; a = umbo ; u = umbonal slope ; i/p = deltidial plates. 



off the coast of Maine) (Fig. 59; see also Figs. 60 and 61) are 

 of very ancient date, and can be accounted for by descent 

 without modification through direct ancestors running back 

 to the early Cambrian time. 



These characters may be enumerated in the following 

 manner. The earliest Brachiopod possessed all the charac- 

 ters essential to each of the following taxonomic divisions^ 

 viz. : 



A. Organisvi. — All the characters which it presents, distin- 



