370 



PALAEOZOIC TIME — CARBONIFEROUS AGE. 



The whole thickness made out by Swallow & Hawn is about 820 feet, and 

 of this 263 feet are called the Upper Permian, and the rest the Lower. Meek 

 & Hayden refer the Lower division, with good reason, and also a part of the 

 Upper, to the Upper Coal measures. The limestones are usually impure, and 

 also magnesian, like most of the limestones of the same region of older date. 

 They are generally rather soft or irregular in structure, and much interlami- 

 nated with clayey or arenaceous beds ; some of the layers contain hornstone. 



II. Life. 



Nothing is yet known respecting the American Permian flora. 



Allowing the Permian the widest range attributed to it, the ani- 

 mal species are rather numerous ; but they include many Carboni- 

 ferous forms, and among them Crinoids, an Archceocidaris, and a 

 Trilobite of «the genus Phillipsia, besides Mollusks. But, restricting 

 the period only to the upper strata, which are recognized by all 

 investigators as Permian, there are only a few Mollusks. 



The species here figured occur in the uppermost beds (Permian of Meek & 

 Hayden). Fig. 611, Monotis Hawni M. & H., cast of the outside of the left 

 valve; 611 a, cast of the interior of the right valve of the same. The genus 

 Monotis is related to Avicula : it has an opening below the beak for the passage 

 of the byssus, as shown in the figure. Fig. 612, Myalina perattenuata M. & H. ; 

 fig. 613, Bakewellia parva ; fig. 614, Pleurophorus subcuneatus M. & H. ; fig. 615, 

 shell of a small undetermined Gasteropod. 



Figs. 611-615. 

 12 



Mollusks.— Figs. 611, 611 a, Monotis Hawni ; 612, Myalina perattenuata; 613, Bakewellia 

 parva ; 614, Pleurophorus subcuneatus ; 615, an undetermined Gasteropod. 



Among the species of Mollusks from the beds referred to the Permian by Swal- 

 low, 75 in number, one-fifth occur also in the Carboniferous beds below. These 

 Carboniferous species are mainly those of wide range, as shown in the following 

 catalogue : — 



