2 FOSSIL REPTILES OP DORSET. 



abound in Amphibia, and are ref errible to the Labyrinthodontic type, 

 which approaches Ophiomorpha in the occasional absence of limbs. 

 Some of the Labyrinthodonts were serpentiform elongated and with- 

 out feet; centra of the vertebrae concave at both ends, jaws and palate 

 armed with teeth, three pectoral plates and a series of small scales 

 covering the ventral region. Nothing certain of the early stages 

 of its life has as yet been found. Some of the Urodela retain the 

 branchiae throughout life ; these are exclusively aquatic and are 

 represented at the present day by Proteus, Siren, and Menolranchus. 



Reptile life which commenced with three genera only in the 

 Permian age, increased to fourteen genera and thirty-one species in 

 the British Trias rocks, of which two only, Plesiosaurus and 

 Ichthyosaurus, passed up through the Jurassic to the Cretaceous 

 beds. The rest did not survive the diminution of heat occasioned 

 by the secular cooling of the earth or other causes. The huge and 

 varied Amphibia fulfilled the role nature had allotted them, and 

 their Reptilian successors in their turn made way for the Mammalia 

 of the tertiary age. The study of Reptilian skeletons is one of 

 very great interest, and intensely increased when extended to a 

 classification of the characteristic features of their ancient forms. 



Reptiles, in common with Amphibia and Pisces, are cold-blooded 

 owing to the arterial and venous blood circulating together through 

 the body and not separately as with warm-blooded animals. The 

 heart has two auricles, but the ventricular chamber is generally 

 incompletely divided ; the red corpuscules of the blood are 

 nucleated, and respiration which is slow and irregular, takes place 

 by lungs and never by branchiae. The skull is articulated to the 

 vertebral column by one condyle. The lower jaw consists of 

 several distinct pieces, and is united to the skull by a bone called 

 the quadrate. In some of the above characters reptiles agree with 

 birds, as in the presence of a single occipital condyle, a complex 

 lower-jaw articulated to the skull by a quadrate bone, nucleated 

 blood corpuscules ; they differ in being cold-blooded, in the absence of 

 air-sacs throughout the body, and in the construction of the fore- 

 limbs, which have never the wing-type. 



