6 ON RHYNCHONELLA ACUTA AND ITS AFFINITIES. 
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. triplicata of Philips 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 thesé 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 re- 
member 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. tripli- 
cata, 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 
difficulties 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 at- 
tained 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 atten- 
tion to one instance of 2. bidens having been recorded as occurring in 
a bed higher than that in which it is usually looked for. 
In Phillips’ “ Geology of Yorkshire,” this marlstone shell, there 
first figured and described, is stated, at page 157, in the list of or- 
ganic remains of the Inferior Oolite, to have been found by Mr. 
* 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. 
