Sauropsida: Class Reptilia 497 



viviparous) they all deposit eggs on land. It is evident that the reproduc- 

 tive mechanism and processes are highly "conservative." Adaptive 

 changes in form of body and in the motor mechanism muscles and 

 skeleton — are made with comparative readiness, but in the reproduc- 

 tive methods there is a marked "lag." 



In the use of the names "turtle," "tortoise," and "terrapin" there 

 is much inconsistency (Fig. 388). The wholly terrestrial chelonians are 

 generally, but not always, called "tortoises." Marine chelonians are 

 "turtles,'* but commercial "tortoise shell" is derived from a marine 

 turtle, the "hawksbill" (Chelonia imbricata). Numerous small chelo- 

 nians, mostly of amphibious habits, are indiscriminately called "tur- 

 tles" or "tortoises." "Terrapin" is applied to various amphibious or 

 aquatic (fresh water or, rarely, brackish) chelonians which some 

 persons esteem highly as food. 



The Chelonia are widely distributed throughout the tropical and 

 temperate zones. The largest living chelonians are found among the 

 sea turtles, some of which weigh hundreds of pounds or even exceed a 

 half-ton. The largest land chelonians are the "giant" tortoises of the 

 ( 'alapagos Islands. 



Early Reptiles 



Geologically, reptiles have been traced back to the early days of 

 amphibians. The earliest definitely recognizable reptilian fossils are 

 found in the Carboniferous Period, when amphibians had already 

 become strongly established (see Table 3, p. 411). They show strong 

 resemblances to the labyrinthodont amphibians, especially in the 

 possession of an extensive system of superficial dermal bones in the 

 head. In fact, some early fossils are difficult of diagnosis as being 

 reptilian or amphibian. If, as the evidence indicates, reptiles were 

 derived from primitive amphibians, in many of which most of the body 

 was covered by heavy dermal bony plates, the occurrence of such 

 structures as gastralia, the dermal bony scales of crocodilians, and the 

 dermal plates of the chelonian shell is quite understandable. 



The amphibians and reptiles were contemporary for long ages, in 

 the course of which the reptiles perfected their adaptations for ter- 

 restrial life and thereby gained over the amphibians an advantage that 

 carried the group to a position of dominance which could never have 

 been achieved by amphibians as such. The Mesozoic Age is commonly 

 called the "Age of Reptiles." The earlier reptiles were terrestrial and 

 comparatively small, but some of the later types were aquatic, with 

 pentadactyl legs transformed into externally finlike paddles. In others, 

 the pterosaurs, a broad, thin fold of skin grew out from each side of the 

 body, supported anteriorly by the pectoral limb and, for the most part, 

 by the greatly elongated fourth digit of that limb, and extending back 



