4 ON RHYNCHONELLA ACUTA AND ITS AFFINITIES. 



extensive series, not suggestive of good and stable specific differences. 

 The variability of character within recognized specific limits, then, 

 being well known, I proceed to lay claim for Rhynchonella acuta to as 

 great an amount of indulgence, in this respect, as for any of its con- 

 geners, referring in evidence to the specimens figured with this paper. 



Its most common form is indubitably that figured by Sowerby 

 and other authors, which occurs in the Lias-marlstone 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 Belgium, &c. ; 

 but this I shall endeavour to show in the sequel to be an immature 

 form, and that, in the succeeding stages, the species to which it 

 belongs attains much larger dimensions, and a higher degree of 

 development, than in the marlstone, although in that stratum it 

 occasionally assumes 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 PL I., 

 Fig. 3, from Churchdown ; and three mesial folds, as in PL I., Fig 2, 

 from Stinchcombe. The scarcity of similar examples is, probably, 

 not so much attributable to their 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 cursory examina- 

 tion will suffice to show that the examples referred to cannot be 

 assigned to any other 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. Rhynchonella acuta. 2. Rhynchonella bidens. 3. Rhynchonella triplicata. 



(Copied from Plate XTTT., Phillips' Geology of Yorkshire.) 



On comparing these with the figures of E. acuta, R. bidens, and R. 

 triplicata, of Phillips, from the marlstone and ironstone-series of 



