314 
THE GEOLOGIST. 
extensive series, not suggestive of good and stable specific differences. 
The variability of character within recognized specific limits, then, beuig 
well knoAvn, I proceed to lay claim for Rliynclionella acuta to as 
great an amount of indulgence, in this respect, as for any of its 
congeners, referring in evidence to the specimens figm-ed with this 
paper. 
Its most common form is indubitably that figured by Sowerby, 
and other authors, wliich occurs in the Lias-niarlstone of Glou- 
cester and Somerset, in the Lias-marlstone and ironstone -series 
of Yorkshire, in France, part of Germany, in the Macigno 
d'Aubange, the equivalent of those deposits in Belgivmi, &c. ; 
biit this I shall endeavour to show in the sequel to be an immature 
fonn, and that, in the succeeding stages, the species to which it 
belongs attains much larger dimensions, and a higher degi'ee of 
development, than in the maristone, although in that stratimi it 
occasionally assmnes characters of importance, as regards our present 
inquiries, which have not hitherto been formally noticed. 
For example, it sometimes presents two mesial folds, as in PI. IX. 
Fig. 3, from Churchdown ; and three mesial folds, as in PI. IX. Fig. 2, 
from Stinchcombe. The scarcity of similar examples is, probably, 
not so much attributable to theu' rarity as to the intractable nature of 
the matrix in which they are imbedded, which renders the extraction 
of its most simple organic forms sufficiently difficult and laborious, 
and that of the more complicated still more so. A cm'sory examination 
will suffice to show that the examples referred to cannot be assigned 
to any c^her species than R. acuta, as they agree perfectly with the 
typical form in lateral outline, and differ from it in no other respect 
than in the number of mesial folds. 
1. 
2. 
3. 
1. Rhynchonella acuta. 
2. Rliynclionella bidens. 
3. Kiynchonella triplicata. 
(Cojiied from Plate XIII., Phillips's Geology of Yorkshire.) 
