REPTILIA. 199 



verse processes. The anterior surface of the neural arches 

 of tlie vertebrte is produced into a process, or " zygosphene," 

 each of which fits into a corresponding cavity, or " zygan- 

 trum," in the hinder surface of the neural arch of the 

 vertebra just in front. 



The three most important groups of the existing Ophidians 

 are the Colubrine Snakes, the Constricting Snakes, and the 

 Viperine Snakes. In the first of these the upper jaws carry 

 solid teeth, with or without canaliculated fangs as well. In 

 the second group are the Boas and Pythons, distinguished 

 by their great size, enormous muscular power, and numer- 

 ous strong recurved teeth. In the third group are Snakes, in 

 which the upper jaws carry only a pair of perforated poison- 

 fangs. 



Most of the existing Snakes are terrestrial in their habits, 

 and are therefore not likely to be preserved in stratified de- 

 posits. Many of these, however, take to the water occasion- 

 ally, and some habitually frequent rivers or the sea itself. 

 All the above-mentioned groups of Ophidians are represented 

 in past time, but they are neither abundant nor of import- 

 ance as fossils. JSTo remains of Ophidians are known to 

 occur in any Palaeozoic or Mesozoic deposit. The earliest 

 known traces of any serpent are in the Lower Kainozoic 

 rocks, one of the oldest being the Palmophis toliapicus of 

 the London Clay of Sheppey. The nearly-allied PalceopMs 

 typliwiis of the Eocene beds of Bracklesham appears to 

 have been a Boa-constrictor-like snake of about twenty feet 

 in length. In the Eocene deposits of North America occur 

 various remains of Snakes. Some of these seem to have 

 frequented the sea (such as TitanopJiis or DinopMs), and to 

 have been large serpents allied to the PalceopMs of the 

 European area. Others, again {Boavus, ZimnopJds, Litlio- 

 pliis), are found in lacustrine strata, and appear to have 

 been related to the Boa-constrictors, but to have been of 

 moderate size. In some of the later Tertiary deposits have 

 been found the poison-fangs of a venomous snake. Upon 

 the whole, however, the Snakes must be looked upon as a 

 comparatively modern group, and not as one of any great 

 geological antiquity. 



