900 NEW YORK STATE MUSEUM 



form. It is thus that the various species have been here deter- 

 mined. However, when the area of the umbo which corresponds 

 to the development of the pyrite form has been determined, it 

 is noticeable that the pyrite form is strikingly smaller than the 

 normal species at a corresponding stage. All of these fos- 

 sils then represent cases of arrested development, with the 

 understanding that the arrest is at no given point but all through 

 the development. The Brachiopoda show the greatest modifi- 

 cation; perhaps because they are the most specialized class 

 and offer less resistance to the. environment. The Cephalopoda, 

 specially the goniatites, show modification in the next degree. 

 Where they are dwarfed, they return to distinctly younger and 

 more primitive appearing types. The Lamellibranchiata and 

 Gastropoda, while often much dwarfed, vary but little in form 

 from their Hamilton ancestors. This may be because they are 

 less specialized groups. Under the unfavorable circumstances 

 here presented, dwarfing is not the unusual but the ordinary 

 phenomenon. 



In considering the descriptions of the various species in the 

 pages to follow, one will observe the frequency with which 

 pyrite forms resemble forms from the Marcellus shales. The 

 Marcellus was another foul water sea; and this seems to 

 represent a recurrence of conditions then existing. The pyrite 

 sea is undoubtedly much more unfavorable to its animal life, 

 but the modifications are in the same direction in enough cases 

 to be noticeable, so that the question is raised, whether a variety 

 of unfavorable conditions does not affect the forms in a similar 

 manner. Herbst's 1 experiments point in that direction, and 

 these faunas also seem to do so. 



A table, showing the species constituting this fauna and their 

 distribution, is appended. 



Descriptions of species 



BRACHIOPODA 



Genus spirifer Sowerby 



Of all the fossils in this group, the spirifers are the most 



abundant, most modified and the most instructive. Though 



found in so close association with a Hamilton fauna, not one 



i 



Loc. cit. 



