8 ON RHYNCHONELLA ACUTA AND ITS AFFINITIES. 
not five, species of that which is one only? It is not pretended by 
naturalists that the difference of a fold or two in shells of this genus 
marks more than a difference of age or of development; hence, as- 
suming the foregone conclusions to be correct, I venture to state that 
neither of the forms noticed by the authorities cited, represents the 
full and perfectly developed condition of this species. 
In a certain portion of the usually unfossiliferous sands near Pains- 
wick, intermediate to those beds in which R. acuta first becomes 
known to us and those in which R. cynocephala disappears, specimens 
occur with the latter, by no means rare, which, on the principles I 
have endeavoured to elucidate, must be referred to the same species. 
These specimens not only attain to a much greater size, but assume 
three or four mesial folds as their most common number. I have not 
found them elsewhere, nor have I seen them in any other collection 
than my own; but this happens probably from their having been 
considered unworthy of retention, on account of the imperfect state 
in which they are usually found. 
I would remark, in conclusion, as lending additional force to my 
observations, that Mr. Hull considers the Terebratula which occurs so 
abundantly in the Frocester and Haresfield beds* with R. cynocephala 
to be identical—and Mr. Davidson has, I believe, adopted this view— 
with 7’. punctata, a shell which occurs in the marlstone associated 
with KR. acuta; we cannot, therefore, be surprized, from their mani- 
fest community of habit, that these two species should have gone 
through time together, from the period of their first creation to that 
of their final extinction. 
I trust that these observations on the natural history of an extinct 
Brachiopod may not have seemed tedious: but I believe every true 
geologist will appreciate any endeavours to clear up errors or con- 
fusion, however small, with regard. to any of his “ time and tide” 
marks ; while the possible demonstration of the wide divergence of 
varieties from their assumed types, as exemplified in this ancient 
species under varied conditions of existence, may not be devoid of in- 
terest to the physiologist in his investigation of phenomena of a 
similar nature. 
* R. rimosa and R. furcellata, also middle Lias shells, have been found in these 
beds by Dr. Wright, of Cheltenham, and myself. 
