DEVONIAN AND MISSISSIPPI AN FAUNAS 107 



with this form at Rockford is Prodromites gorbyi which occurs also in 

 the Chouteau hmestone of central Missouri. This latter goniatite is 

 the most advanced one of the Mississippian faunas, having, as it does, 

 a secondarily lobed suture such as, at no very distant period in the past, 

 was considered to be characteristically Mesozoic in type. Another 

 peculiar cephalopod in the fauna is Trihloceras digonum which occurs 

 in the fauna at various localities. A peculiar type of pelecypod is 

 found in the genus Promacrus, which occurs also in the early Mississip- 

 pian beds of Belgium. These and many other forms in the fauna char- 

 acterize it as something distinctly younger than any Devonian fauna, 

 with numerous bonds of affinity uniting it with the higher and more 

 typical Mississippian faunas. However, there occur associated with 

 these characteristic portions of the fauna certain species, especially 

 among the pelecypods, which are clearly Devonian derivatives, and, 

 strange to say, their relationships are usually with members of the 

 Hamilton fauna, rather than with the higher Devonian faunas of the 

 Eastern Continental Province. The Hamilton relationships of the 

 fauna are perhaps best seen in the fauna of the Glen Park limestone,' 

 where the pelecypods and gastropods are all close allies of Hamilton 

 forms, and where one form even seems to be specifically identical, but 

 associated with these is a member of the highly characteristic Missis- 

 sipian genus Syringothyris.^ 



The origin of this southern Kinderhook or Chouteau fauna is 

 believed to have been in the Atlantic Basin, where Middle Devonian 

 faunas of Hamilton type had probably retreated as the Upper Devo- 

 nian immigrants became established in the Eastern Continental Prov- 

 ince, or where they had persisted during Upper Devonian time, having 

 never been encroached upon by the immigrants. During the long 

 lapse of time most of the species had been modified, and there had 

 been absorbed into the fauna a new element from some unknown 

 region. The return of this fauna into the Mississippi Valley Basin 

 marks the opening of the Kinderhook epoch and the Mississippian 

 period. 



1 Weller, Trans. St. Louis Acad. ScL, XVI, 435-71. 



2 The species described in the Fauna of the Glen Parh Limestone {loc. cit). as 

 Spirijer jejfersonensis, has since been definitely identified as a member of the genus 

 Syringothyris. 



