CHAP. XVIII PHYLOGENY AND ONTOGENY 5O9 



PHYLOGENY AND ONTOGENY 



Wherever successive stages in the life history of an individual 

 resemble in important anatomical features the adult individuals 

 of other species occurring in successive members of a strati- 

 graphical series, the development of the individual may be 

 regarded as an epitome of the development of the species ; it 

 also generally throws light on the origin and relationships of 

 allied genera and families. 



In the case of the fossil Brachiopoda comparatively little 

 work has yet been done in tracing their ontogeny or phylogeny, 

 though the abundance, variety, and excellent state of preserva- 

 tion of the extinct species offer a promising field for investiga- 

 tion. It is to Dr. C. E. Beecher and other recent American 

 palaeontologists that we owe our advance in this branch of the 

 subject. 



In the first place, in about forty genera, representing nearly 

 all the leading families of the group, the important fact has been 

 established of the presence of a common form of embryonic shell, 

 termed the " protegulum," which is " semicircular or semielliptical 

 in shape with a straight or arcuate hinge line and no hinge 

 area " (Beecher).^ Its minute size and delicate texture cause its 

 preservation to be rare, but its impression is not uncommonly 

 left on the beak of the adult shell. 



The main features of this embryonic shell are exhibited in the 

 adult Lower Cambrian Brachiopod Oholus (Kutorgiroa) lahradorictis 

 (Billings) ; the sub-equal semielliptical valves have lines of growth 

 running concentrically and parallel to the margin of the shell, 

 and ending abruptly against the straight hinge line ; and this 

 indicates that there has been no change in the outline and 

 proportions of the shell during its stages of growth, but only a 

 general increase in size. It is very significant that we have 

 here a mature type possessing the common embryonic characters 

 of a host of widely separated genera, and we may therefore regard 

 it as the most primitive form known. 



Many genera pass through this so-called " Paterina " stage 



either in the case of both their valves, or more generally in the 



case of the dorsal valve only ; but modifications in the form of 



the protegulum arise, which are due to the influence of accelerated 



^ Amcr. Jour. Science, 1890-1893. 



