ANATOMY OF THE POISON GLAND OF HELODERMA. 19 



teeth, from the base of which, I may add, they are separated by the outer wall 

 of the dental sacs. These are cup-shaped folds of mucous membrane com- 

 pletely surrounding the basal portions of the teeth. Immediately external to 

 them is a shallow groove which is limited at the outer side by the prominent 

 fold of mucous membrane mentioned by Stewart as intervening between the 

 lip and the lower jaw. This fold is connected to the jaw by a series of vertical, 

 obliquely transverse folds which lie in the regions between the teeth. They 

 interrupt the continuity of the groove already mentioned and subdivide it into 

 a series of shallow depressions, into the more anterior of which the ducts of the 

 poison gland open. It would seem probable that these depresssions serve as 

 temporary reservoirs for the secretion. The tips of the upper teeth project 

 into these depressions when the mouth is closed (see fig. 4). 



The teeth of Heloderma have been fully described by Cope (1900). Both 

 upper and lower teeth are grooved, although no evidence of a poison gland has 

 been found in connection with the upper teeth, and even in the lower teeth the 

 poison ducts are associated only with the anterior three or four pairs. The 

 latter show no features distinctly differentiating them from the remaining 

 teeth. The grooves are formed by projecting folds of enamel which extend 

 along both the anterior and posterior edges of the tooth. The anterior fold is 

 the stronger and its groove correspondingly deeper. The secretion is doubt- 

 less carried along the groove by capillarity. The teeth are hollow within 

 when dry, but there is no sign of a perforation at the apex. In life the internal 

 cavity is filled by a fleshy papilla arising from the jaw margin. Most of the 

 teeth are firmly united to the bone, but the younger teeth are quite free and 

 easily detachable. 



Shufeldt has described a tendinous expansion arising from the outer sur- 

 face of the superficial muscles close to the hinder margin of the mandible and 

 spreading out over the gland. Posteriorly it is rather narrow and strong, but 

 anteriorly, as its fibers diverge, it expands to form a thin sheet closely adherent 

 to the gland. Shufeldt is inclined to believe that, although it contains scarcely 

 any muscle fibers, by its contraction the venom of the gland can be ejected 

 through the ducts. As to the nature of this contraction he says nothing, but 

 I think it may be considered as purely passive, due to the tensions produced 

 in it by the movements of the jaw. Such movements produced artificially in 

 the dead animal do, as a matter of fact, cause ejection of the venom. I may 

 add to Shufeldt's account by stating that the superficial muscles from which 

 the tendinous expanse arises are on the outer side of the posterior half of the 

 mandible. From this point the fibers pass forward and downward and then 

 bend inward under the lower jaw in precisely the position for the movements of 

 the jaw to produce the tensions mentioned. In fig. 1, on the right side of 

 the figure, this part of the tendinous expansion can be seen arising from the 

 muscles immediately behind the angle of the lower jaw. Anteriorly it has been 

 cut away along with the skin in order to show the poison glands. 



Concerning the homology of the poison gland with the mouth glands of 

 other vertebrates, it is obvious from its relative position that it is not com- 



