368 



PALEOZOIC TIME. 



is related to Avicula: it has an opening below the beak, for the passage of the byssus, 

 as shown in the figure. Fig. 688, Myalina pevattenuata M. & H. ; Fig. 689, Bakewellia 

 parva M. & H. ; Fig. 690, Pleurophorus subcuneatus M. & H. ; Fig. 691, shell of a small 

 undetermined Gasterqpod. 



Figs. 687-691. 



687a 



Mollusks. — Fig3. 687, 687 a, Pseudomonotis Hawnii ; 688, Myalina perrattenuata ; 



wellia parva ; 690, Pleurophorus subouneatus ; 691, an undetermined Gasteropod. 



Bake- 



Among the species of Mollusks from the beds referred to the Permian by Swallow, 

 75 in numb«r, nine tenths occur also in the Carboniferous beds below. 



III. General Observations. 



We observe the following facts connected with the period : (1.) 

 The beds are apparently all marine strata, for the fossils are marine^ 

 (2.) The numerous alternations, between impure limestones and clays 

 and some sand deposits, indicate oscillations through the period in the 

 depth of water, between moderate depths and very shallow waters. 

 (3.) The absence of coal beds is proof that there were no fresh-water 

 Carboniferous marshes in the regions where the rocks have thus far 

 been examined. (4.) The non-occurrence of these marine strata over 

 the region east of the Mississippi seems to show that this eastern part 

 of the continent was dry land. Early in the Carboniferous period, 

 the Pennsylvania region was raised, and became dry even of its old 

 marshes ; for only the Lower Coal-measures occur there ; and, in the 

 Permian period, as it appears, the dry region had extended so as to 

 include all the country east of the Mississippi. (5.) The beds occur 

 within the same region, or on the borders of the same region, in 

 which the Coal-formation during the Carboniferous period was repre- 

 sented by limestones ; that is, in the great interior sea which had so 

 long existed as the Paleozoic representative of the Gulf of Mexico, — 

 a comparatively shallow, but extensive, inland sea, stretching north- 

 ward. The present western limit of the Gulf is nearly in a north-and- 

 south line with the western boundary of the State of Kansas. The 

 existence of these Permian deposits was, then, owing to a continuation 

 of the conditions that characterized the Carboniferous period. That 



