Ch. XX.] 



LOWER PURBECK. 



389 



inhabited by placentals, for Australia now supports one hundred and 

 sixty species of marsupials, while the rest of the continents and 

 islands are tenanted by about seventeen hundred species of mamma- 

 lia, of which only forty-six are marsupial, namely, the opossums of 

 North and South America. But the great difference of age of the 

 strata in each of these three localities seems to indicate the pre- 

 dominance throughout a vast lapse of time (from the era of the 

 Upper Trias to that of the Purbeck beds) of a low grade of quadru- 

 peds ; and this persistency of similar generic and ordinal types in 

 Europe while the species were changing, and while the fish, reptiles, 

 and mollusca were undergoing vast modifications, would naturally 

 lead us to suspect that there must also have been a vast extension in 

 space of the same marsupial forms during that portion of the sec- 

 ondary epoch which has been termed " the age of reptiles." Such an 

 inference as to the wide geographical range of the marsupials of the 

 olden time has been confirmed by the discovery in the Trias of North 

 America of three lower jaws of a quadruped allied to Myrmecobius. 

 It was found by the late Dr. Emmons in beds probably coeval with 

 the " Keuper " of Europe. The predominance in earlier ages of these 

 mammalia of a low grade, and the absence at present of species of 

 higher organization, is certainly in favor of the theory of progressive 

 development. 



Beneath the freshwater strata to which the mammaliferous marl 

 belono-s is a thin band of greenish shales, with marine shells and im- 

 pressions of leaves, like those of a large Zostera, forming the base of 

 the Middle Purbeck. 



Lower Purbech. — Beneath the thin marine band last mentioned, 

 purely freshwater marls occur, containing species of Cypris (fig. 375, 

 a, 6), Valvata, and Limncea, dif- 

 ferent from those of the Middle Fig. 375. 

 Purbeck. This is the beginning 

 of the inferior division, which 

 is about 80 feet thick. Below 

 the marls are seen, at Meup's 

 Bay, more than 30 feet of brack- 

 ish-water strata, abounding in a 

 species of Serpula, allied to, if 

 not identical with, Serpula co- 

 acervites, found in beds of the 

 same ag-e in Hanover. There 



are also shells of the genus Rissoa (of the subgenus Hydrobia), and a 

 little Cardium of the subgenus Protocardium, in these marine beds, 

 together with Cypris. Some of the cypris-bearing shales are strangely 

 contorted and broken up, at the west end of the Isle of Purbeck. 

 The great dirt-bed or vegetable soil containing the roots and stools of 

 Cycadece, which I shall presently describe, underlies these marls, and 

 iests upon the lowest freshwater limestone, a rock about 8 feet thick, 



Cyprides from the Lower Purbecks. 



a. Cypris Purbeckensis, b. Cypris punctata, 



E. Forbes. E. Forbes. 



