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THE GEOLOGIST. 



Professor Quenstedt, in like manner, in his recently- piiblislied 

 "Jura," treating of R bidens, Phillips, seems disposed to consider it 

 and E. triplicata as varieties of R. acuta, distinctly pointing out their 

 general resemblances ; although German authors generally agree in 

 stating "that this species does not occur in the Suabian Alps. 



Not to needlessly multiply quotations, I may yet briefly state that 

 Davidson, in his Monograph on British Jurassic Brachiopoda, figures 

 what he considers to be the R bidens and R. triijlicata of Phillips as 

 varieties of R variabilis, which he believes to range through the 

 Lower and Middle Lias but, unless his figures are taken from the 

 original specimens of Professor Phillips (and these were very ill 

 drawn by the last-named gentleman), I discover nothing in his obser- 

 vations upon either species to modify the conclusions at which I have 

 arrived from a comparison of the observations of all these authors. 



Labyrinthine as appears to be the confusion of ideas in the state- 

 ments cited, the clue appears to me to lie within grasp. Let us 

 remember that the three forms, in one case, occupy the same zone, 

 occasionally occurring together, at other stages or places one or other 

 numerically preponderating ; that the difference between R. acuta 

 and R bidens, in the opinions of authors and observers in every way 

 entitled to respect, is no greater than between the latter and R. tri- 

 plicata, being one of degree only and not of kind. The suggestion, 

 therefore, naturally arises that they may really pertain to one species. 

 Assuming that we are justified in arriving at this conclusion, all 

 diflftculties vanish. We simply learn the not uninteresting facts in its 

 natural history — that the geographical distribution of one of the most 

 characteristic shells of the stage to which it belongs was co-extensive 

 with that of many of its usual companions, from which it would 

 otherwise appear to be somewhat unaccountably separated ; and that, 

 in particular portions of the area which it occupied, it attained to 

 degrees of development denied to it in others. 



Having pointed out instances of the confusion of these varieties with 

 species of lower stratigraphical range, I will now direct attention to 

 one instance of R. bidens having been recorded as occurring in a bed 

 higher than that in which it is usually looked for. 



In this monograph, Mr. Davidson gives his reasons from considering R. bidens 

 and its synonym R. triplicata, as specifically different from R. acuta. His statements 

 on this subject should be referred to.— En Geol. 



