COMMENCEMENT OF MAMMALIA. 59 



dilians — and the pliosaurus, a creature which appears to 

 have formed a link between the plesiosaurus and the cro- 

 codile. We know of at least six species of the flying 

 saurian, the pterodactyle, in this formation. 



Now, for the first time, we find remains of insects, an 

 order of animals not well calculated for fossil preservation, 

 and which are therefore amongst the rarest of the animal 

 tribes found in rocks, though they are the most numerous 

 of all living families A single libellula (dragon-fly) was 

 frund in the Stonefield slate, a member of the lower 

 oolitic group quarried near Oxford ; and this was for 

 several years the only specimen known to exist so early : 

 but now many species have been found in a corresponding 

 rock at Solenhofen, in Germany. It is remarkable, that 

 the remains of insects are found most plentifully near the 

 remains of pterodactyles, to which undoubtedly they 

 served as prey. 



The first glimpse of the highest class of the vertebrate 

 sub-kingdom — mammalia — is obtained from the Stones- 

 field slate, where there has been found the jaw-bone of a 

 quadruped evidently insectivorous, and inferred, from pe- 

 culiarities in the structure of that small fragment to have 

 belonged to the marsupial family, (pouched animals.) It 

 may be observed although no specimens of so high aclass 

 of animals as mammalia are found earlier, such may ne- 

 vertheless have existed : the defect may be in our not hav- 

 ing found them ; but, other things considered, the pro- 

 bability is, that heretofore there were no mammifers. 

 It is an interesting circumstance, that the first mammifers 

 found should have belonged to the marsupialia, 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 clearly a link between the oviparous vertebrata 

 (birds, reptiles, and fishes) and the higher mammifers. 

 This is further established by their possessing a faint 

 development of two canals passing from near the anus to 

 the external surface of the viscera, which are fully pos- 

 sessed in reptiles and fishes, tor the purpose of supplying 

 aerated water to the blood circulating in particular vessels, 

 but which are unneeded by mammifers. Such rudiments 

 of organs in certain species which do not require them in 

 any degree, are common in both the animal and vegetable 



