i68 WATER REPTILES OF THE PAST AND PRESENT 



SNAKES 



The chief differences between snakes and lizards have already 

 been given and need not be repeated, save very l)rictly. Snakes 

 are always functionally legless, though some have vestiges of the 

 hind pair; the brain-case is wholly bony; the upper temporal bar 

 is wanting; the lower jaws are united in front by ligaments only, 

 like those of the mosasaurs; the vertebrae are greatly increased in 

 number, and always have the additional zygosphenal articulations 

 like those of Clidastcs and Mosasaiirus and some lizards; there is 

 but one lung, and the eyes are always without free eyelids. But 

 these characters are really not very important, since every one of 

 them is found in the lizards or mosasaurs, except the complete 

 ossification of the brain-case, and even this is partly ossified in 

 the mosasaurs. It is rather the presence of all these characters 

 which distinguishes a snake from a lizard. 



The number of living snakes is nearly as great as that of the 

 living lizards, and their distribution over the earth is very similar. 

 Snakes are for the most part strictly terrestrial in habit. Some 

 live more or less among trees, and some live in the water, though 

 with but few exceptions all are fully capable of rapid progres- 

 sion upon land. They are almost invariably carnivorous in 

 habit, swallowing their prey whole, and usually alive, as has 

 been described. Some poison their prey or crush it to death before 

 swallowing it. Some feed upon eggs which are swallowed whole 

 and then crushed in their stomachs by projecting bones from the 

 under side of the vertebrae developed for that purpose. In size 

 snakes vary from a few inches in length to twenty-five or more 

 feet, no known extinct forms being larger than the living anacondas 

 and boas. In geological history the earliest remains known date 

 from the latter part of the Cretaceous, and it is quite probable that 

 they have a briefer history than that of the lizards of which they 

 are the descendants. Venomous serpents are known only from 

 comparatively recent geological times, and it is probable that 

 venomosity is the latest and final specialization of importance in 

 the reptilian class. 



Of strictly aquatic snakes there is no known geological history, 

 and it is improbable that there is an\' such history. There are 



