ADAPTATIONS 109 



generate, derivatives from the ancestors of the true turtles. If 

 the former view be true, we have evidence of another structural 

 adaptation to an aquatic life ; for in both the Chelonidce and the 

 Dermochelyidce the temporal region of the skull, unlike that of 

 ordinary chelonians, is roofed over with bone, and if this has 

 been developed independently in two pelagic groups, it must 

 evidently be an aquatic adaptation. In any case, the latter 

 conclusion is supported by the fact that a similar structure, 

 although in a minor degree, is developed in the fresh-water 

 chelonians known as snappers which constitute the family 

 Chelydridce. In the last-named family, as well as in the fluvi- 

 atile soft-tortoises {Trionychidce)> the adaptation to an aquatic 

 existence has not been carried sufficiently far to have modified 

 the limbs into paddles ; the members of both groups spending 

 much of their time on land. 



In this place may be mentioned a curious modification oc- 

 curring in some recent and in a group of extinct reptiles, which 

 has evidently been independently acquired, and would seem to 

 be adaptive, and perhaps connected with aquatic habits, 

 although this is by no means certain. Moreover, the modifica- 

 tion in question is not structurally identical in the two cases, 

 being formed in the one entirely by dermal tissues while in the 

 other the vertebral column is involved. One of the most re- 

 markable of the American Iguanidcz is the basilisk, or sail- 

 backed iguana (Basiliscus americanus), in the males of which 

 the back and the middle line of the upper surface of the tail 

 carry a tall sail -like crest, supported by flexible rods, and fully 

 equal in height to the depth of the body. These basilisks, like 

 other iguanas, habitually live in trees, preferring, however, 

 those which overhang water, into which, when alarmed, they 

 throw themselves, and swim away with strong strokes of the 

 limbs. From the fact of the crest in all the species (for there 

 are several) being confined to the males, it has been urged that 

 it has nothing to do with swimming ; but until we know defin- 

 itely whether the males are better swimmers and more prone 

 to take to the water than the females, this conclusion cannot 

 be accepted without hesitation. Possibly the structure in 

 question may be merely an ornamental appendage of the male 

 sex, like the large comb of a cock. Be this as it may, it is dif- 



