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 (Proteroglypha), 



or behind a row of hooked 



teeth ( Opisihoglypha) . 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 the flesh of the 



prey. Simultaneously the r> 



secretion of the poison gland, 



which is forced out by the 



pressure caused by the con- 



traction of the temporal FlG - 637. Head of Calopeltls Aesculapii. a, Dorsal 

 T i i , " n view, b, ventral, c. Lateral view of head of 

 muscles, IS injected UltO the TropidonotH. viperinus (after E. Schreiber) , 



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



wound ; it is thus mixed with 

 the blood, and quickly causes 

 the death of the victim. 



The hard structures of the 



terior nasal scute; d, anterior nasal scute; 

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

 labial scutes ; h, nasal scute ; i, prseorbital 

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

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



integument, which have the scutes; . q ' mental scute; r > cervical scutes ; 



s, cervical scales; t, ventral scutes. 



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. n. 14 



