424 Vertehrata. 



base with a poison gland* behind the head, which is to be 

 regarded as a specially developed buccal gland. In the mucous 

 membrane of the mouth, behind the poison fang, there are several 



replacement teeth at various stages 



. of development. When the tooth is 



— ■ — ^,^ \ not in use it is covered by a fold of 



C k ^'^V the mucous membrane : it is brought 



~rim^— — -^l!1~^ forward by a movement of the 



;'/ maxilla, with which it is immovably 



/,'4--ka connected. In the Vipers the maxilla 



y^y is very short and edentulous, except 



o' for the poison fangs ; in the Protero- 



Fig. 349. Poison gland and glypha it bears several other small, 



poison fang of a Snake; dia- simple teeth. In one OTOup of Snakes, 



grammatic. fc gland, g duct of the "^ £ i • 



same, ia poison canal, o upper, o' lower one or more 01 the posterior 



opening.— Ong. maxillary teeth is provided with an 



open groove on the anterior side : 

 grooved teeth. It has been stated that these teeth (always ?) 

 are likewise connected with poison glands, and that small animals 

 die soon after being bitten by them. 



A few examples of this large group are given below. 



1. ..Giant snakes i^ero'podLa) exhibit rudiments of the hind limbs in the 

 form of a small claw-like process on either side of the anus. Teeth simple. 

 Large snakes belong here : Python (up to about 10 ni. long), "several species in 

 Asia and Africa ; the females incubate their eggs (the warmth of the body 

 while brooding considerably exceeds that of the environment) : JSoo constrictor, 

 in South Amei-ioa, up to about 6 m. 



2. The Coluhridx, hke those which foUow are destitute of posterior 

 a,ppendages; teeth simple. In Britain, the Ringed Snakes {Tropidonotus 

 natrix), easily recognised by two large yellow patches on the back of the head : 

 ^sculapius' Snake (C ^sculapii) is probably the species venerated by 

 the ancient Romans. Numerous forms in warm countries. 



3. The Opisthoglypha possess in the posterior portion of the maxilla one or 

 several gi-ooved teeth. The Whip-snakes (Dryophis, etc.), a group indigenous 

 to the Tropics ; distinguished by their extraordinarily long, thin body and pointed 

 head ; they Uve in trees. 



4. The Proteroglypha are venomous snakes, which have, anteriorly, in the 

 maxilla a poison gland with a fine gi-oove on the front side (cf. Pig. 348 C — -D)) 

 and behind this, small simple teeth. 



a. The o b r a {Naja tripudians) can spread out the skin behind the head 

 into a wide hood, for the anterior ribs are tiu'ned outwards ; a pair of spectacles 

 is represented on this hood ; 2 m. long; India. The Coral snake (Elaps) is 

 ringed with black and red ; small; in South America. Several other genera 

 in wai'm countries. 



6. The Sea Snakes (genus Hydrophis, etc.), specially characterised by 

 the very compressed tail and the very small scale-Uke ventral splints ; numeroiis 



* The poison gland opens quite freely on the wall of the mouth, as does the upper 

 aperture of the poison fang ; the two apertures, however, lie close together, with the 

 edge of that of the gland round the opening in the tooth, so that the poison cannot 

 flow into the mouth (cf. Fig. 349). 



