FCETAL MEMBRANES 719 



mother and unborn young. Its embryonic part is chiefly formed from 

 a union of the serous or subzonal membrane and the allantois, but in 

 some cases the yolk-sac and the subzonal membrane form a provisional 

 placenta. The placenta establishes a vital union between the embryo 

 and the mother. 



Now it is interesting to notice that there are some hints of placental 

 connection in animals which are much lower than Mammals. In 

 some species of Mustelus and Carcharias there is a connection between 

 the yolk-sac and the wall of the uterus ; in the Teleostean Anableps 

 the yolk-sac has small absorbing outgrowths or villi ; in Trachydosaurus 

 and Cyclodus among Lizards, the vascular yolk-sac is separated from 

 the wall of the uterus only by the porous and friable egg-shell. In 

 Clemmys among Chelonians, there is an absorbent protrusion of the 

 foetal membranes. In Birds also, there are hints of yolk-sac vilh 

 which absorb volk, and of allantoic villi which absorb albumen. 



Extinct Reptiles 



The first known occurrence of fossil Reptiles is in Per- 

 mian strata ; in the Trias most of the orders or classes are 

 represented ; while the " golden age " of the group was 

 undoubtedly during Jurassic and Cretaceous times. 



Some of the modern Reptiles are linked by a series of 

 fine gradations to very ancient progenitors— the Crocodiles 

 of to-day lead back to those of the Trias, the New Zealand 

 Sphenodon to the Triassic Rhynchocephalia ; but we have no 

 example of a Reptilian genus which has persisted from age 

 to age as Ceratodus has done among Fishes. Among the 

 fossil forms we find " generalised " types, which exhibit 

 affinities with groups which in our classification of recent 

 forms may be very widely separated. 



The following types of extinct reptiles seem to have 

 entirely disappeared : — 



Theromorpha. — Lizard-like terrestrial animals with limbs adapted 

 for walking; found in the Permian and Trias. The group shows a 

 remarkable combination of reptilian and mammalian characters. In 

 illustration of reptilian characters we may note the pineal foramen, the 

 complex lower jaw, usually articulating with a firmly fixed quadrate, 

 the usual presence of pre- and post -front als. Mammalian features are 

 illustrated in some types by the differentiation of the teeth into incisors, 

 canines, and molars ; by a single temporal arcade like a zygomatic 

 arch ; by the way the limbs raise the body oft the ground ; by the 

 union of the pelvic bones into an os innominatum (pubes and ischia 

 forming a stout ventral symphysis) ; by the reduction of the quadrate ; 

 by the share the squamosal may take in forming the articulation for 

 the lower jaw. 



