RHYXCHOCEPHALIA. 329 



birds. The cleavage is meroblastic, and the embryo is provided 

 with an amnion and allantois. The amnion is a purely em- 

 bryonic structure, but the allantois is the cloacal bladder which is 

 precociously developed and enormously enlarged as the embry- 

 onic respiratory organ. 



Re^Dtiles are cold-blooded. In the cold and temperate regions 

 they fall into a kind of winter sleep, and in hot climates there is 

 a summer sleep which comes to an end with the beginning of the 

 rainy season. Most of them are very tenacious of life and can 

 exist a long time without food and with limited respiration. 

 The power of reproducing lost parts exists (e.g. the tail in lizards), 

 but is less than in Amphibia. 



They first make their appearance in the Lower Permian 

 {Protorosaurus). In the Secondary Period they obtained an enor- 

 mous development both in variety of form and in size. In the 

 Tertiary Period th'ey declined. There are about 3,500 living 

 species at present known. They are divided into nine sub- 

 classes, the interconnections of which are somev\'hat compli- 

 cated. They may be arranged as follows : — 

 Sub-class 1. Bhynchocephalia . Permian to present day. 

 ,, 2. Lepidosauria. 



Dolichosauria , Cretaceous . 



Mosasauria, Cretaceous. 



Lacertilia, Jurassic to present day. 



Ophidia. Cretaceous or Eocene to 

 present day. 



Triassic to present day. 



Triassic to Cretaceous. 



L. Jurassic to Cretaceous. 



Triassic to Cretaceous. 



Triassic to Cretaceous. 



Permian and Triassic. 



Triassic to present day. 



Sub-class 1. RnVNCHOCEPIlALTA.* 



Lizard-like creatures ivitk biconcave vertebrae, immoveable qiiad- 



* A. Giinther, " Anatomy of Hatteria," Phil. Trans. 167, 18G7, p. 595. 

 G. Osawa, a series of papers on the anatomy of Hatteria in Arch. f. mic. 

 Anat., vols. 47, 1896, p. 570 : 49. 1897, p. 113 ; 51. 1898. p. 481 ; 52, 1898, 

 p. 268. F. Siebenrock, Znr Osteologie dt^s Hatteria-Kopfes, Sitzb. Akad. 



