﻿272 T. Bavichon — What is a Brachiopod ? 



is no modification, that I can see, of any of tlie forms that con- 

 tinue beyond, as far as the Brachiopoda appear to be concerned, 

 and why should a number of genera, such as Lingula, Discina, Crania, 

 and Rhynchonelln, have continued to be represented with the same 

 characters and often with but small modification in shape during tlie 

 entire sequence of geological strata ? Why did they not offer 

 modifications or alter during those incalculable ages ? Limiting 

 myself to the Brachiopoda, let us see what further they will tell us 

 upon this question. Taking the present state of our knowledge as 

 a guide, but admitting, at the same time, that any day our conclusions 

 and inductions may require to be modified by fresh discoveries, let 

 us ascertain whether they reveal anything to suj)port Darwinian 

 ideas. We find that the larger number of genera made their first 

 appearance during the Paleozoic periods, and since they have been 

 decreasing in number to the present period. We will leave out of 

 question the species, for they vary so little that it is often very 

 difScult to trace really good distinctive characters between them ; it 

 is different with the genera, as they are, or should be, founded on 

 much greater and more permanent distinctions. Thus, for example, 

 the family SpiriferidcB includes genera which are all charac- 

 terized by a calcified spiral lamina for the support of the brachial 

 appendages ; and however varied these may be, they always retain 

 the distinctive characters of the group from their first appearance to 

 their extinction. The Brachiopodist labours under the difficulties of 

 not being able to determine what are the simplest, or which are the 

 highest families into which either of the two great groups of his 

 favourite class is divided ; so far then he is unable to point out any 

 evidence favouring pi-ogressive development in it. But, confining 

 himself to species, he sees often before him great varietal changes, so 

 much so, as to make it difficult for him to define the species ; and it 

 leads him to the belief that such groups were not of independent 

 origin, as was universally thought before Darwin published his great 

 work on the Origin of Species. But in this respect the Brachiopoda 

 reveal nothing more than other groups of the organic kingdoms. 



It would appear that the earliest forms among the Brachiopoda 

 are referable to the division Tretenterata, which includes the genera 

 Lingulella, Lingula, Discina, and Oholella. Of these only Lingula 

 and Discina have lived on with but slight modifications in external 

 shape during the entire sequence of geological time ; and they are 

 still represented by several species. But in rocks somewhat later 

 in age (from the middle beds of the Menevian group or Lower Lin- 

 gula flags) to those in which the above genera are found, there occurs 

 a species of Ortliis (0. LTiclcsii), which may possibly be the first re- 

 presentative, as far as we are awaxe, of the division Clistenterata. 

 On this point, however, I would refer to Professor King's note pre- 

 viously given. Since the Cambrian period both divisions continue 

 to V)e represented without apparently showing a tendency to pass one 

 into the other. Now although certain genera, such as Terehratula, 

 Jthi/nchonella, Crania, and Discina, have enjoyed a very considerable 

 geological existence, there are genera, such as 8tringocephalus, Uncites, 



