466 On Progressive Development (Spr. 
Setting aside the consideration of the order in which the de- 
velopment of the invertebrate classes has proceeded, as embracing too 
wide a field and requiring a minute investigation of the anatomical 
relations of this vast class of animals, the cold-blooded vertebrata 
appear to offer the most convenient opportunity of observing the ana- 
logies which subsist between animals of the same type of conforma- 
tion, but differing in the degree of perfection at which their various 
systems of organs have arrived. 
It appears that at one period of the earth’s history—that in which 
the deposition of the secondary formations was taking place,—circum- 
stances were highly favorable to the development of the cold-blooded 
tribes of vertebrata. The oceans swarmed with enormous cephalopoda, 
with gigantic individuals of a saurian race which has long since vanish- 
ed from the surface of the globe, but whose remains scattered in such 
profusion through the oolitic group furnish the zoologist with data 
which enable him to fill up many apparent vacuities in the scale of the 
creation. 
Nor, as might have been expected, if we determine to admit the 
present as the only true standard by which to judge of the state of 
things in past epochs, was this form of organization chiefly peculiar to 
the inhabitants of the waters ; the ancient continents contained animals 
of this type only ; the megalosaurus and the iguanodon peopled the 
forests; the banks of the rivers and fresh-water lakes were frequented 
by crocodiles and huge salamanders, while the pterodactyli pursued 
their prey amid the palms, the cycadee, and the tree ferns, of the 
primeval Flora. But not until after the deposition of the great cal- 
careous formation do we find any trace of the existence of a warm- 
blooded animal: not even the most strenuous advocates for the unifor- 
mity of the past and present operations of nature have been able to 
prove that animal life had progressed so far as the development of the 
class mammalia, or of birds, until after the epoch just alluded to. 
The only exception to be made with regard to this statement is met 
with in three or four specimens consisting of fragments of the lower 
jaw of an animal which has been pronounced by the highest authority to 
have been a species of didelphis. This fossil, discovered in the Stones- 
field slate, a member of the oolitic series, lying below the cornbrash 
and above the Bath oolite, contains nine similar acuminated molares, 
terminating in three elevated points; but as no living didelphis 
possesses this number. of molar teeth on one side of the jaw, and as 
those of the didelphis present the characters of insectivorous teeth, 
