(Ringueberg on the Niagara Shales. 265 
the railroad cut just above Lewiston; at both of which places it 
presented its peculiar uneven surface and hard texture. 
The following Niagara species may be added to the list there 
given: Retepora asperato-striata^Plaiyceras angulatmn., Sptri- 
fcra crispa^ Retzia apriitis^ and a small undetermined rhynchon- 
elloid shell of which a few specimens have been found in the 
shales. Orthis lynx must be removed to the list of species 
common to the Clinton and Niagara, as it has since been found 
in the shales of the latter. Here also may be added Orthis ele- 
gantula. Three determined species are characteristic of the 
group itself, although judging from the material at hand there 
are many more. 
Though this group was regarded as forming a part of the 
Clinton, the weight of the palaeontological evidence proves it to 
be more closely allied to the Niagara, and confirms the propriety 
of classifying it as a distinct member of that group of equal 
value with the others composing it. 
It evidently represents a period of comparatively rapid physi- 
cal change during which the ancestors of the Niagara forms 
(which had been developing elsewhere, synchronously with 
the forms representing the latter part of the Clinton here) were 
suddenly introduced, and mingling with the Clinton species 
then extant, produced this transitional group. 
Various causes might be assigned as the active agents in the 
production of this change; but the one which seems most prob- 
able is a general subsidence of the whole region with concom- 
mitant inflowing currents bringing with them an abundance of 
life from adjacent seas in which a Niagara or Niagara-like 
fauna was flourishing. 
This complete change of both physical and organic surround- 
ings is a sufficient explanation of the complete extinction of the 
Clinton forms except such as were by nature readily capable of 
adapting themselves to the new enforced phase of existence. 
This same cause would also have its influence on the newly 
introduced forms, and of these again only such as easily became 
acclimated and adapted, would survive; especially if the change 
continued, as is probable, after the initiatory subsidence. For 
immediately following this period we find a complete and abrupt 
change in the nature of the sedimentary deposit. Now let us 
