360 REPTILIA. 



ORDER II. OPHIDIA. The second order of Reptiles is 

 that of the Ophidia, comprising the Snakes and Serpents, and 

 distinguished by the following characters : 



The body is always more or less elongated, cylindrical, and 

 worm-like, and whilst possessing a covering of horny scales, is 

 always unprovided with a bony exoskeleton. The dorsal ver- 

 tebrae are concave in front (proccelous), with rudimentary trans- 

 verse processes. There is never any sternum, nor pectoral 

 arch, nor fore-limbs, nor sacrum, and as a rule there are no 

 traces of hind-limbs. Rudimentary hind-limbs, however, are 

 occasionally present (e. g., in Python and Tortrix). There are 

 always numerous ribs. The two halves or rami of the lower 

 jaw are composed of several pieces, and the rami are united 

 anteriorly by ligaments and muscles only, and not by cartilage 

 or suture. The lower jaw, further, articulates with the skull by 

 means of a quadrate bone (fig. 312, a), which is always more 

 or less movable, and is in turn united with the squamous por- 

 tion of the temporal bone ("mastoid bone"), which is also 

 movable, and is not firmly united with the skull. The superior 

 maxillae are united with the praemaxillae by ligaments and mus- 

 cles only, and the palatine arches are movable and armed with 

 pointed recurved teeth. Hooked conical teeth are always pre- 

 sent, but they are never lodged in distinct sockets or alveoli. 

 Functionally, they are capable of performing nothing more 

 than merely holding the prey fast, and the Snakes are provided 

 with no genuine masticatory apparatus. The heart has three 

 chambers, two auricles and a ventricle, the latter imperfectly 

 divided into two cavities by an incomplete septum. The lungs 

 and other paired organs are mostly not bilaterally symmetrical, 

 one of each pair being either rudimentary or absent. 



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 numerous 

 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 occasionally, 

 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 importance as fossils. 

 No remains of Ophidians are known to occur in any Palaeozoic 



