1833.] in the cold-blooded Vertebrata. “473 
perfected in the family of the Crocodilida, where the most complete 
system of respiration obtains among reptiles. 
It has now been shown, that in the two great systems which exer- 
cise the most important influence over the development of the animal, 
there is a gradual and well-marked progression towards perfection in 
the organs by aid of which these functions are performed ; and that 
it does not require the aid of the imagination to trace the steps by 
which the simple air-sac of the sturgeon has passed through the inter- 
vening stages in the proteus and the triton to the elementary lung 
of the frog and the more perfect organ of the crocodile. 
It will be as easy to show that the same system of gradual progression 
has been followed throughout all the members of the series: the links 
which unite Batrachia with the Ophidian reptiles, and these latter with 
the Saurian tribes, are too evident to render necessary a lengthened 
detail. 
In the genus cecilia we are supplied with the form which connects 
the Batrachia with the serpent race. The auricle presents a partial 
septum, an indication of the change to be effected in the heart of the 
true serpents, where there are three distinct cavities. In the arrange- 
ment of the teeth upon the maxillary and palatine bones, the cecilia 
resembles the proteus, but in the shape of the teeth comes nearer the 
true Ophidia. 
With regard to the respiratory organs, the left lung is, as in ser- 
 pents, retarded in development. The skin is soft and naked as in Ba- 
trachia, but according to Baron Cuvizr, it contains, within its substance, 
small scales regularly disposed in transverse bands. The true serpents 
are separated from the Saurian reptiles by the total absence of any ves- 
tige of sternum or extremities; this is the most prominent character, 
and will therefore be the most easily traced. 
If we passed at once from Ophidia, where extremities are totally 
wanting, to the lizards where they are perfectly formed, we might sup- 
pose that there had been a sudden production in one order, of an organ, 
of which we had observed no elementary condition in the order imme- 
diately preceding ; a circumstance entirely at variance with all that 
has hitherto been observed. 
But in this instance, as in every other, there have been successive 
stages through which the organs of locomotion have passed. There is 
a small family of reptiles placed between Sauria and Ophidia, in whom 
these organs are seen to be gradually developed. In the anguis and 
the ophisaurus a rudimentary sternum and pelvis are concealed beneath 
the integuments; in the scheltopusik a small femur has been added, 
2 P 
