70 REPTILES 



the snake-like form of the body in the mosasaurs, coupled with 

 the weakness of the pelvis, and the extreme length of the neck 

 in the plesiosaurs (which would appear to be exceedingly incon- 

 venient to an animal on land), suggest a completely aquatic life 

 for the members of those two groups. If this be a true infer 

 ence, there can be little doubt that they were viviparous. 



One other adaptive modification in the ichthyosaurs ma)' 

 be mentioned. From their " coprolites " it is known that they 

 were furnished with a spiral valve to the intestine ; a feature 

 paralleled by sharks, rays, and chimaeras among fishes. Ichthyo- 

 saurs also resembled sharks and many dolphins in possessing 

 a dorsal and a caudal fin ; the latter being, like that of sharks 

 (and unlike that of dolphins), vertical. 



There are remarkable differences in the shape of the eggs 

 of reptiles, some being spherical, while others are elliptical, with 

 the two ends symmetrical ; in no case, however, do we find the 

 typical " egg-shape," that is to say, one end rounded, and the 

 other more or less pointed. Crocodiles and gharials lay ellip- 

 tical eggs about the size of those of a goose ; but the tuatera's 

 eggs are spherical. Pythons' eggs are also spherical, while those 

 of many other snakes are elliptical, and fastened together in 

 bunches or strings. Many land-tortoises, again, lay spherical 

 eggs, which in the case of the giant species are about the size of 

 lawn-tennis balls ; and the eggs of the marine turtles are simi- 

 larly shaped. On the other hand, the fresh-water terrapins and 

 batagurs lay elliptical eggs. The season for this diversity in 

 shape is difficult to discover, seeing that it has no correlation 

 with terrestrial or aquatic habits. 



As already mentioned, most reptiles leave their eggs to 

 take care of themselves after they are laid. The sand-lizard 

 (Lacerta agilis), for example, deposits its eggs, which usual ly 

 range from five to eight in number, during July in a depression 

 in sandy soil, where they are left to hatch by the aid of mois- 

 ture and the sun's heat. In cases, however, where sand is not 

 to be found the eggs are deposited in leaves, vegetable mould, 

 or rubbish. In the case of the green lizard (L. viridis) the 

 eggs, generally eight to ten in number, are developed for five 

 weeks within the maternal oviducts, and are then laid in suit- 

 able situations, where they remain four weeks more before they 

 hatch. The ring-snake usually selects decaying vegetable 



