[Illustration: Fig. 2. Lateral view of the left maxillary of _Tantalophis discolor_ (Günther). (KU No. 40143). × 17.]
A thin and otherwise small parotid gland or "venom sac" extends posteriorly from beneath the eye to about the angle of the jaw; a minute duct connects with the anteromedian surface and extends to the fleshy part of the mouth at the posterior end of the maxillary (Figure 3).
[Illustration: Fig. 3. Lateral view of the head of _Tantalophis discolor_ (Günther), showing the position and relative size of the parotid gland. (KU No. 40143). × 3.]
_Hemipenis._--In _situ_ the hemipenis extends to the posterior edge of the thirteenth caudal. The unforked part of the organ is bedecked with numerous heavy longitudinal folds alternating with thinner folds. The basal parts of the two heads are covered with moderate sized spines, those closest to the base and the sulcus being the smallest. The distal parts of the heads are covered with calyces. The sulcus bifurcates on the unforked part of the organ at a point about two-thirds of the distance from the base to the division of the organ. The sulcus is a deep groove between heavy folds proximally and is a shallower furrow distally (Figure 4).
[Illustration: Fig. 4. Hemipenis of _Tantalophis discolor_ (Günther). The organ was cut on the ventral surface and opened. (KU No. 40143). × 4.]
_Relationships._--Using Dunn's (1928) arrangement of the American colubrid snakes as a guide permits the taxonomist to group _Tantalophis_ with several genera, some of which occur in South America and others in the West Indies. Although the significance of such generic characters as scale pits and nature of the hemipenis is not clear, these characters must, of necessity, be utilized in attempting to ascertain the relationships of _Tantalophis_ to other colubrid snakes. Assuming that the primary divisions of the American colubrids into subfamilies based on the nature of the sulcus spermaticus and the presence or absence of hypapophyses on the posterior vertebrae have some reality, _Tantalophis_ must be placed in the subfamily Xenodontinae comprising genera chiefly South American in their distribution, but with several genera in Middle America and a few in North America and the West Indies. In order to limit the number of genera to be compared with _Tantalophis_, only those xenodontines having apical pits and bifurcate hemipenis are considered. These include _Cyclagras_, _Drepanoides_, _Hypsirhynchus_, _Ialtris_, _Leimadophis_, _Pseudablabes_, _Siphlophis_, _Tachymenis_, _Tomodon_, and _Trypanurgos_. Aside from differences in scutellation, _Leimadophis_, _Siphlophis_, and _Trypanurgos_ have the heads of the hemipenes terminating in a disc, and _Ialtris_ has a plicate hemipenis. _Tomodon_ has basal spines on the hemipenis. The hemipenes of the other genera have proximal folds, distal spines, and distal calyces, not greatly unlike the condition found in _Tantalophis_. Of these, _Cyclagras_, _Hypsirhynchus_, and _Pseudablabes_ have round pupils and certain differences in scutellation. _Drepanoides_ and _Tachymenis_ have elliptical pupils like those of _Tantalophis_, but _Tachymenis_ has only one apical pit, and _Drepanoides_ has one apical pit or none. In the above characters no especially close relationship between _Tantalophis_ and any one of these genera is apparent.