

3io 



SYSTEMATIC ZOOLOGY 



crocodiles. A copulatory organ or penis is present ; in the 

 lizards and serpents there are two penes, one on each side of 

 the median line of the cloaca ; in the turtles and crocodiles 

 there is only one. 



Most reptiles are oviparous, but some lizards and snakes are 

 viviparous. The eggs resemble birds' eggs ; they have a large 

 amount of yolk and are inclosed in a membranous or calcareous 

 shell. In the course of development a membrane grows around 

 the embryo, completely inclosing it (Fig. 317). This membrane 



Fig. 318. Rhampho- 

 rhynchus, an extinct 

 flying reptile ; re- 

 stored. (After Zittel, 

 from Parkerand 1 las- 

 well's Text-book.) 



Fig. 319. Tguanodon bernissartensis, one sixtieth natural size. 

 co, coracoid ; is, ischium ; />, pubis (pectineal pro< post? 



pubic process (pubis); 1 [V, I-V, digits. (After Dollo, from 

 Parker and HasweU's Text-book.) 



is called the amnion, and the space between it and the embryo 

 is filled with a fluid, the amniotic fluid. This amnion occurs 

 in the development of the birds and the mammals also, and 

 hence these three groups of the vertebrates are often called 



