I will now consider these characters in connection with variations found in a series of specimens of A. pulchra and A. nigra.
1. The shape of the head and snout is subject to some variation in both A. pulchra and A. nigra. Unless the difference in shape in the type of A. texana is very great, one is safe in ignoring it as a basis of specific distinction.
2. One of my specimens of A. nigra (Cal. Acad. Sci. No. 6255) shows a horizontal suture extending from the nostril to the second labial. Another (No. 6244) has such a suture between the nostril and the rostral plate.
3. There is considerable variation in the shape and size of the frontal plate in both A. pulchra and A. nigra. It not infrequently is twice as broad as long (No. 6236, etc.), but may be nearly as long as broad. Sometimes it nearly touches the rostral (No. 5103).
4. The anterior supraocular is nearly as broad as the distance which separates it from its fellow in some specimens of A. pulchra (No. 5110) and A. nigra (Nos. 6233, 6243, 6249, etc.). In some specimens it has scarcely more than half this breadth.
5. I regard the plates which Mr. Boulenger calls interparietal and occipital as frontoparietal and interparietal, respectively. The former plate is not completely divided in any of my specimens. One example of A. pulchra (No. 5110), however, has it longitudinally divided throughout the posterior third of its length. The interparietal (occipital of Mr. Boulenger) is completely divided longitudinally in one example of A. nigra (No. 6228) and divided through one-fifth its length in another (No. 6218).
6. This is the normal arrangement, but is subject to variation.
7. This shield may be absent.
8. The number of lower labials ranges from five to seven.
9. The number of scales around the middle of the body varies in A. pulchra from twenty-four to thirty-four, while in 54 specimens of A. nigra the number is twenty-eight in 12, thirty in 36, and thirty-two in 6.