TENTH JURASSIC PERIOD. 



388. The close of the last period was marked by the final 

 extinction of at least 36 generic forms. The four generic forms 

 of reptiles which appear for the first time in this ninth period 

 are the Stenosaurus, Streptospondylus, Emys, and Platemys, of 

 which the Stenosaurus is peculiar to the period and characteristic 

 of it, not re-appearing in the next or any subsequent period. Of 

 the reptile genera of former periods, those which re-appear in the 

 present, and finally became extinct at its close, are the Plesio- 

 saurus, Teleosaurus, and Pliosaurus. 



Independently of the species of the other classes which lived in 

 this period, 186 species of mollusca and radiata have been cata- 

 logued and described by M. D'Orbigny. 



389. The seas were circumscribed by nearly the same limits, 

 retiring nevertheless a little within their former shores. They 

 were inhabited by nearly the same assemblage of genera, the 

 species being, however, altogether different. The cephalopods 

 were not numerous, the prevailing marine genera consisting of 

 gastropods, and lamellibranchia. 



390. The land, independently of some revived genera, was 

 inhabited by the saurian reptiles Stenosaurus and Streptosaurus, 

 and by the tortoise forms, denominated Emys and Platemys, all 

 of which were probably amphibious. 



No vegetable remains of this period have been found. 



TENTH JURASSIC PERIOD. 



391. The deposits of this period correspond with the Portland 

 beds. They lie generally upon the Kimmeridge strata of the 

 preceding period. 



392. A general view of the fauna, with the previous exceptions, 

 is given in the table on the next page. 



Of the mollusca and radiata 63 species have been described. 



393. The seas of this period, like those of the former, preserved 

 the general configuration of their outlines, still retiring a little 

 within their former shores. The marine fauna among fishes was 

 increased by the Meristodon, and among mollusca by the Cyclas. 

 The principal saurians of the last period disappeared from the 

 coasts, and were replaced by the Cetiosaurus. This marine reptile 

 must have rivalled the existing whale in magnitude, some speci- 

 mens indicating a total length of 40 to 50 feet. The bones found 

 in some strata in England, of somewhat more recent date, may 

 have been carried up from the sea along the bed of a river by the 

 tide, or the animal when living, as is often at present the case 

 with existing cetaceans, may have wandered up the river, and 

 dying, have been deposited in its bed. 



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