Ch. XX.] FOSSILS OF THE MIDDLE PURBECK. 295 



series. It will be seen that Cypris fasciculata (fig. Fi s- 838 - 



337, b) has tubercles at the end only of each valve, a 



character by which it can be immediately recognized. In 



fact, these minute crustaceans, almost as frequent in some 



of the shales as plates of mica in a micaceous sandstone, 



enable geologists at once to identify the Middle Purbeck 



in places far from the Dorsetshire cliffs, as, for example, in T / > ^ s ? BristovU 



r . . '.,.,. l E. Forbes. Middle 



the Vale of Wardour, m Wiltshire. Thick siliceous beds Purbeck. 



of chert occur in the Middle Purbeck filled with mollusca and cyprides of 

 the genera already enumerated, in a beautiful state of preservation, often 

 converted into chalcedony. Among these Professor Forbes met with 

 gyrogonites (the spore-vessels of Ckarce), plants never until 1851 discov- 

 ered in rocks older than Eocene. In a bed of this series, about 20 feet 

 below the " Cinder," Mr. W. R. Brodie has lately found (1854), in Dur- 

 dlestone Bay, portions of several small jaws with teeth, which Professor 

 Owen, after clearing away the matrix, recognized as belonging to a small 

 mammifer of the insectivorous class. The teeth with pointed cusps re- 

 semble in some degree those of the Cape Mole ( Chrysochlora aurea) ; 

 but the number of the molar teeth (at least ten in each ramus of the 

 lower jaw) accords with that in the extinct Thylacotherium of the Stones- 

 field Oolite (see below, Chap. XX,). This newly-found quadruped, there- 

 fore, seems to have been more closely allied in its dentition to the 

 Thylacotherium than to any existing insectivorous type. As in Thylaco- 

 therium, the angular process of the jaw is not bent inwards, an osteologi- 

 cal peculiarity confined to the marsupial tribes (see Chap. XX.), and 

 Professor Owen therefore refers the Spalacotherium to the placental or 

 ordinary class of monodelphous mammalia. 



In a former edition of this work (1852), after alluding to the discovery 

 of numerous insects and air-breathing mollusca in the " Purbeck," I re- 

 marked that, although no mammalia had then been found, -• it was too 

 soon to infer their non-existence on mere negative evidence." The 

 scarcity of the remains of warm-blooded quadrupeds in Oolitic rocks, and 

 the fact of none having yet been met with in deposits of the Cretaceous 

 era, may imply that there were few mammalia then living, and their 

 limited numbers may possibly have some connection with the enormous 

 development of reptile life in all Secondary periods, as compared to Ter- 

 tiary or Recent times. If so, the phenomenon has at least no relation to 

 an incipient or immature condition of the planet, as some have imagined, 

 for, so far from being characteristic of primary or even older secondary 

 times, it belongs to the Maestricht chalk, the newest subdivision of the 

 cretaceous series, and that too in a manner even more marked than in 

 the older oolitic rocks. Nevertheless in the present imperfect state of our 

 information respecting the land-animals of the Cretaceous and Jurassic 

 periods, exclusively derived from marine and fluviatile strata, and our 

 total ignorance of the deposits formed in lakes and caverns at the same 

 date, it would be premature to attempt to generalize on the nature of so 

 ancient a terrestrial fauna. 



