williams.] THE EVOLUTION OF THE KHYNCHONELLAS. 61 
derberg fauna, it becomes evident that we hav r e here an expression of 
acceleration in development along the very lines of normal evolution, 
and associated with favorable conditions of environment, which are 
expressed in the vigorous fertility and growth of the individuals of 
the race. 
This leads to the remark that the described distinctions, which make 
it appear that there were very few, if any, camarotoechias in the Lower 
Helderberg faunas, though they were abundant below and above, are 
not, properly speaking, generic characters, but are mutations express- 
ing temporary adjustment of the organisms to the environmental con- 
ditions, which in the next stage were dropped. To use these names 
as they are applied by the authors, the evidence makes it probable not 
only that the Uncinulus of the Lower Helderberg descended from the 
camarotoechias of the earlier faunas, but that the camarotoechias of 
the following Oriskany were in direct line of descent from the same 
genus, Uncinulus. 
This interpretation implies, of course, that these characters, which 
are taken by the authors to be of generic value, are really plastic 
characters, which are variable in the whole race of rhynchonellas, and 
that their particular expression is a reflex of environmental conditions 
and is not an inherited characteristic. The truth of this proposition 
is confirmed by studying other forms. 
When the Oriskany species are critically studied, not only is great 
development observed, but also great plasticity of the same parts. 
The spirifers, the rensselasrias, the orthids, and the strophomenoids 
are all conspicuous not only for their great size but for the great 
development of their parts about the interior of the beak, hinge, and 
surfaces of muscular attachments; and not only this, but a series of 
Spirifer arenosus exhibits great plasticity in the relative strength of 
development of internal features, not only in series living together 
and of the same size and age, but in the degree of development at cor- 
responding stages of growth on different individuals. This plasticity 
is not confined to Spirifer; the other vigorous species, rensselserias and 
orthids, show the same feature. 
Another fact points to the probability that the taxonomic importance 
of these characters, as at present defined, is overdone, if not altogether 
erroneous. In their most valuable revision of the Brachiopoda, Hall 
and Clarke state that the genus Rhynchonella is, from a literary point 
of view, rightly restricted to forms of the type R. loxia, none of which 
are known to occur at horizons older than the Mesozoic. Neverthe- 
less, when it comes to distributing, in this same literary way, the 
rhynchonellids of the Paleozoic under some name, ninety-five American 
Paleozoic species still have to be called Rhynchonella. 1 The most 
patent reason for this inconsistency is that in these ninety-five species 
1 Vide Schuchert. 
