64 THE TRIAS AND OOLITE. 



isting whales. Here, too, we find the true crocodilia largely 

 developed, and five genera have been described, (Teleosaurus, 

 Steneosaurus, Cetiosaurus, &c.) The two first are like croco- 

 diles of our own time in all respects, except a somewhat greater 

 bulk, and certain peculiarities, indicating more aquatic 

 habits. The last derives its name from the approximation to 

 the whale tribes seen in the form of its vertebrae. In this 

 group there is a genus presenting ball-and-socket vertebrae, 

 and thus proving its advanced character ; but, strange to say, 

 the concavity is in this case directed backwards, instead of 

 forwards, which is the universal arrangement in similar cases, 

 in our era. 



The first glimpse of the highest class of the vertebrate sub- 

 kingdom Mammalia is obtained from the Stonesfield slate, 

 where there have been found several specimens of the lower 

 jaw-bone of a quadruped evidently insectivorous, and inferred, 

 from peculiarities of structure, to have belonged to the marsu- 

 pial family, (pouched animals.) ( 3S ) It may be observed, 

 although no specimens of so high a class of animals as mam- 

 malia are found earlier, such may nevertheless have existed : 

 the defect may be in our not having found them ; but, other 

 things considered, the probability is that heretofore there 

 were no mammifers. It is an interesting circumstance that 

 the first mammifers found should have belonged to the mar- 

 supialia, when the place of that order in the scale of creation 

 is considered. In the imperfect structure of their brain, 

 deficient in the organs connecting the two hemispheres and 

 in the mode of gestation, which is only in small part uterine 

 this family is usually regarded as only a little advanced above 

 the character of the bird. 



The highest part of the oolitic formation presents some 

 phenomena of an unusual and interesting character, which 

 demand special notice. Immediately above the upper oolitic 

 group in Buckinghamshire, in the vicinity of Weymouth, 

 and other situations, there is a thin stratum, usually called by 

 workmen the dirt-bed, which appears, from incontestable evi- 

 dence, to have been a soil formed, like soils of the present day, 

 in the course of time, upon a surface which had previously 



