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 perattenuota M. & H.; Fig. 689, Bakewellia 

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

 undetermined Gasteropod. On Permian Reptiles, see page 851. 



Figs. 687-691. 



687a 



Mollusks.— -Figs. 687, 687 a, Pseudomonotis Hawnii ; 688, Myalina perrattenuata ; 689, Bake- 

 wellia parva ; 690, Pleurophorud subcuneatus ; 691, an undetermined Gasteropod. 



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

 75 in number, nine tenths occur also in the Carboniferous beds below. 



m. 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 occurrence of these marine strata over the 

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

 the continent was still undergoing oscillations. Early in the Car- 

 boniferous period, eastern Pennsylvania may have been raised, and 

 made dry land ; for only the Lower Coal-measures occur there ; 

 but this was, apparently, not true of the southwestern part of the 

 State or of West Virginia. (5.) The loestern beds occur within 

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

 the Coal-formation during the Carboniferous period was represented 

 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 the Permian deposits was owing to a continuation 

 of the conditions that characterized the Carboniferous period. That 



