THE EXOSKELETON. 



209 



are traversed by a canal ; the canal is connected with the duct of a 

 poison gland, and through it the secretion of the latter is poured. 

 Frequently the maxilla is much reduced, and contains on each 

 side only one large perforated poison-tooth, near which, however, 

 other larger and smaller supplementary teeth are always placed 

 (Solenoglypha). The grooved teeth are rarely more numerous, and 

 are attached to the maxillaries either quite in front (Proteroylypha), 

 or behind a row of hooked 

 teeth ( Opisthoglypha) . In both 

 cases the maxilla is larger than 

 that of the Solenoglypha. In a 

 the Aglyphodonta, however, 

 where there are no grooved 

 teeth, the maxilla attains the 

 greatest size and the richest 

 dentition. While the grooved 

 teeth are immovably fixed, 

 the tubular poison teeth are 

 erected, with the maxillaries 

 to which they are attached, 

 when the mouth is opened, and 

 are, when the snake strikes, 

 driven into ^.he flesh of the 

 prey. Simultaneously the c 

 secretion of the poison gland, 

 which is forced out by the 

 pressure caused by the con- 

 traction of the temporal FlG - 637. Head of Calopeltis Aetculapii. a, Dorsal 



view, 6, ventral, c, Lateral view of head of 



muscles, is injected into the 

 wound ; it is thus mixed with 

 the blood, and quickly causes 

 the death of the victim. 



The hard structures of the 

 integument, which have the 

 form of scales, scutes, and 

 splints, vary much in form, number, and arrangement. While 

 the dorsal surface of the trunk is always covered with smooth or 

 keeled scales; the head is covered with scales as well as with 

 scutes and plates, which, like those of the Lizards, are distin- 

 guished according to their special position as frontal, parietal, and 

 occipital scutes ; also as rostral, nasal, temporal, and labial scutes, etc. 



VOL. II. 14 



Tropidonotus viperinus (after E. Schreiber). a, 

 Frontal scute; b, supra-ciliary scutes; c, pos- 

 terior nasal scute ; d. anterior nasal scute ; 

 e, parietal scute ; /, rostral scute ; g, upper 

 labial scutes ; A, nasal scute ; i, prseorbital 

 scutes ; k, loreal scute ; I, postorbital ; m, tem- 

 poral scutes ; o, chin scute ; p, lower labial 

 scutes ; q, mental,, scute ; r, cervical scutes ; 

 s, cervical scales ; t, ventral scutes. 



