PERMIAN PERIOD. 529 



group varies a good deal, but may amount to 3000 or 4000 

 feet. 



2. The Middle Permians, consisting, in their typical de- 

 velopment, of laminated marls, or " marl-slate," surmounted 

 by beds of magnesian limestone (the " Zechstein" of the Ger- 

 man geologists). Sometimes the limestones are degenerate or 

 wholly deficient, and the series may consist of sandy shales 

 and gypsiferous clays. The magnesian limestone, however, of 

 the Middle Permians is, as a rule, so well marked a feature 

 that it was long spoken of as the Magnesian Limestone. 



3. The Upper Permians, consisting of a series of sandstones 

 and shales, or of red or mottled marls, often gypsiferous, and 

 sometimes including beds of limestone. 



In North America, the Permian rocks appear to be confined 

 to the region west of the Mississippi, being especially well de- 

 veloped in Kansas. Their exact limits have not as yet been 

 made out, and their total thickness is not more than a few 

 hundred feet. They consist of sandstones, conglomerates, 

 limestones, marls, and beds of gypsum. 



LIFE OF THE PERIOD. The Permian Rocks have yielded 

 a very considerable number of plants, most of which are speci- 

 fically distinct from those of the Coal-measures. Though the 

 species, however, are distinct, many of the Permian genera 

 date back to the antecedent Carboniferous period. Thus, 

 besides several genera of Carboniferous Ferns, the Permian 

 Rocks contain the well-known genera Lepidodendron and Cala- 

 mites. The Sigillarioids, however, seem to have finally dis- 

 appeared. Conifers are by no means uncommon, and some 

 of these ( Ullmanid) produce true cones. The genus Walchia 

 comprises the most characteristic of the Permian Conifers. 



The Protozoa are represented in the Permian deposits by a 

 few Sponges and Foraminifera. The Coelenterates are repre- 

 sented by Corals, but these are rarely abundant. The Rugosa 

 are reduced here to the single genus Polycoelia. Crustaceans 

 are also by no means largely represented. The Trilobita 

 have disappeared, as have the Eurypterids. The King-crabs 

 (Limulus} are, however, represented by one species. Ostracoda 

 are tolerably abundant, and the genus Prosoponiscus has been 

 regarded as referable to either the Isopoda or the Amphipoda. 

 Lastly, the genus Hemitrochiscus has been founded for the 

 reception of a Permian fossil which has been regarded as the 

 remains of a true Crab. If this determination be correct, the 

 tribe of Brachyurous Decapods has its commencement here. 



Molluscs occur in greater abundance than any of the pre- 

 ceding. The genera Fenestella and Acanthodadia represent the 

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