XVIII PHYLOGENY AND ONTOGENY 



509 



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 investi- 

 gation. 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 semi- 

 elliptical 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 Oholiis ( Kutorgi7ia) lahra- 

 doricus (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 



1 A7ner. Jour. Science, 1890-1893. 



