ao. 2382. SEXUAL DIFFERENCES IN COLORATION— BLAKE. 465 



ward into a blotch which occupies most of the space between the 

 sides of the lower jaw. The only exceptional specimen is the male 

 just mentioned, which has well-developed throat markings. 



5. The anal opening is situated much nearer the base of the tail in 

 the females than in the males. This character and character No. 2 

 above are those commonly used for the discrimination of the sexes 

 in museum material, but the measurements which I have made of 

 the series of alcoholics in the National Museum show that there is 

 some overlapping in this character. In 15 males the total length of 

 the tail, measured in a straight line along its underside from the 

 notch in the anal plates to its tip, varied from 30 to 43 mm. (in an 

 exceptional case only 23 mm.) and averaged 36.8 mm. The dis- 

 tance from the notch in the anal plates to the middle of the anal 

 opening varied from 11 to 16 mm. (in two cases only 8 mm.) and 

 averaged 12.7 mm.; the length from the middle of the anal opening 

 to tip of tail varied from 20 to 28 mm. (in two exceptional cases 15 

 and 17 mm.) and averaged 24 mm. In the 27 females measured the 

 tail varied from 20 to 37 mm. (with a single exceptional specimen of 

 40 mm.) in length, with a average of 29.6 mm.; the preanal measure- 

 ment was 2 to 7 mm. (in two exceptional specimens 10 mm.), with 

 an average of 5 mm.; and the postanal length 15 to 33 mm., with 

 an average of 24.6 mm. From these figures it will be seen that the 

 tail of the male averages 36.8 mm. in length, that of the female 29.6 

 mm.; its preanal length in the male averages 12.7 mm., in the female 

 5 mm.; while the postanal portion of the tail is essentially the same 

 in both sexes (24 mm. in males, 24.6 mm. in females). There is 

 some overlapping in each of these measurements, which is least in 

 the case of preanal length of the tail. The shape of the tail is subject 

 to considerable variation in both sexes. On the whole the tail of the 

 female is perhaps more slender than that of the male, but the varia- 

 tion seems too great to make this difference of any diagnostic value. 



6. The female has a well-developed supra-auricular line of yellow 

 or orange spots, which are usually aggregated into a streak. In the 

 male this is much less developed, except in the case of the abnormal 

 specimen, to which reference has already been made. A few of the 

 more weakly marked females, however, are not distinguishable in 

 this regard from the more heavily marked males. 



7. The female has almost invariably a few yellow spots on the 

 crown in front of a line connecting the posterior corner of the eyes. 

 Of the 24 males examined only about 8 shoAved one or two such 

 spots. It may be noted that the abnormal male already referred 

 to had no spots in this region. 



The distinctive characters just described may be summarized for 

 the sexes thus: 



27177— 21— ProaN.M. vol.no 30 



