SEX AND REPRODUCTION 69 



which most of the boas produce living young, while pythons 

 lay eggs. A still greater difficulty arises (as in the case of La- 

 cliesis and Lacerta) when we find the two modes of reproduction 

 occurring respectively in different species of the same genus. 

 The North American banded water-snake (Tropidonotus fas- 

 ciatus) belongs, for example, to the same genus as the 

 egg-laying ring-snake, and yet produces living offspring. 



In some instances among lizards all or most of the 

 members of a family may be oviparous or viviparous as the case 

 may be ; but in other instances there is a strange mingling of 

 the two types of reproduction. The iguanas, for example, 

 appear to be, at least for the most part, oviparous, while the 

 skinks are as markedly viviparous. On the other hand, the 

 above-mentioned viviparous lizard stands out as a marked 

 exception among an oviparous group ; while in the family 

 Anguidce we find that whereas the continental glass-snake, or 

 scheltopusik {Ophisaurus) lays eggs, the British slow-worm 

 (Anguis) gives birth to living offspring. 



All crocodiles, together with chelonians and the tuatera, are 

 oviparous. 



Of the modes of reproduction of fossil reptiles we are, from 

 the nature of the case, in most instances ignorant. By what 

 may be regarded as a fortunate accident, we know, however, 

 that the ichthyosaurs produced their young alive, after the 

 manner of sea-snakes. The evidence for this is afforded by 

 certain skeletons (of females) from the German Lias, within the 

 ribs of which are enclosed the remains of foetuses. This de- 

 parture from the normal type of (oviparous) reproduction is 

 evidently correlated, as in sea-snakes, with more or less com- 

 pletely pelagic habits ; and thus indicates, as might have been 

 expected from the structure of their paddles, that the typical 

 ichthyosaurs never came ashore, but led lives similar to those 

 of whales ; the pelagic habit being most developed in the 

 specialised Ophthalmosaurus of the Oxford Clay. 



Whether plesiosaurs, pelagic crocodiles, and sea-serpents 

 (mosasaurs) were likewise viviparous, or whether, like turtles, 

 they came ashore to lay their eggs, cannot be positively stated. 

 On the one hand, the less specialised type of paddle (as com- 

 pared with the ichthyosaurs) might lead to the inference that 

 these reptiles occasionally came to land. On the other hand, 



