The Saint Peter Sandstone. 



79 



General Discussion. 



The species are, therefore, distributed paleontologically 

 as follows : 



MOLLUSC A — Lamellibranchiata- 



Gasteropoda- 



Cephalopoda — 

 MOLLUSCOIDEA— Brachiopoda- 



Bryozooa- 

 PORIFERA— ? 



-Cypricardites, 

 Modiolopsis, 

 Tellinomya, 

 Holopea, 

 Murchisonia, 

 Ophileta, 

 Platyceras, 

 Pleurotomaria , 

 Orthoceras, 

 Crania, ? 

 Lingula, 

 Orthis, 

 donbtful, 

 cf. Rau fella. 



4 species. 

 7 species. 

 2 species. 

 2 species. 



2 species. 

 1 species. 

 1 species. 

 1 species. 



3 species. 

 1 species. 

 1 species. 

 1 species. 

 1 species. 

 1 species. 



Total, 14 genera, 28 species. 



All the species of the Saint Peter sandstone are. very sim- 

 ilar to others in the overlying Trenton limestone. In fact, if 

 all had been found in some stratum of the lower part of the 

 Trenton (Galena) series instead of in the Saint Peter forma- 

 tion, their occurrence would not have been thought surpris- 

 ing. The similarity with Trenton species is far greater than 

 the lithologic difference of the two formations had led 

 geologists to expect. But the Saint Peter fauna is only 

 molluscan, i. e., there are wanting in it the Coelenterata, 

 Bryozoa, Crinoidea and Trilobita which are so abundant in 

 the Trenton. These conditions seem to prove that a physical 

 much more than a time break separates the two formations, 

 for the species are but little different while the faunas as a 

 whole are widely different. A comparison with the forma- 

 tion that underlies the Saint Peter, shows an opposite 

 relation. The Shakopee fauna, so far as the writer has 

 found, consists of a few species of Gasteropoda and Cepha- 

 lopoda. It is like that of the Saint Peter sandstone in being 

 molluscan, but the individual species are different, being of 

 the Upper Calciferous type. It can be argued from these 

 considerations that the Saint Peter is separated from the 

 Shakopee rather by a time gap than by physical changes. 

 The paleontological evidence is not to be exclusively relied 



