346 STRUCTURE OF THE VERTEBRATES 



sac, but the allantoic bladder is well developed in the amphib- 

 ia. Under the conditions of a large yolk the ventral body wall 

 would not interfere with the continued growth of the allantois, 

 and its enlargement into a breathing organ would be possible. 

 It is probable that the first reptiles laid their eggs in moist 

 places with the yolk surrounded by a gelatinous capsule. It is 

 certain that the typical embryological membranes were de- 

 veloped before the recent reptilian sub-classes were separated 

 from the original stock. 



D. Radiations of the Reptiles 



Seymoiiria, of the Lower Permian Period, an era when the 

 amphibia were still the dominant type of vertebrate life, is the 

 most primitive reptile known. The skull is wide and flat, the 

 brain cavity very small in proportion to the total size, the poste- 

 rior margin is indented by an otic notch across which the 

 tympanum was stretched, and the dorsal bones retain the 

 primitive arrangement. In the vertebrae both an intercentrum 

 and neurocentrum are present, the former being relatively large, 

 the latter having short dorsal arches. The ribs articulated with 

 both inter- and neurocentrum, and become progressively smaller, 

 extending on past the sacrum to the tail vertebrae. A single 

 sacral rib is enlarged for the attachment of the pelvis. 



The more primitive characters of the first reptiles were soon 

 lost. The skull became deeper, the ribs were less numerous, the 

 intercentrum was smaller, and the typical two sacral ribs be- 

 came attached to the ilium. These reptiles were lacking fenestrae 

 in the dermal covering of the skull, openings through which the 

 muscles of the jaw became attached to the dorsal surface of the 

 skull roof. This group forms the stem of reptile evolution. The 

 turtles are the. only living representatives of the group, the Sub- 

 class Anapsida. 



Two other sub-classes sprang from the early anapsid reptiles, 

 the Parapsida and the Synapsida. The former have a single pair 

 of dorsal fenestrae, and a deep notch on the margin of the skull 

 between the maxilla and the quadrate. The lizards and snakes 

 are the living representatives of the parapsid reptiles. 



Synapsid reptiles had a single lateral fenestra in the dermal 



