A MONOGRAPH OP THE SEA-SNAKES (HYDROPHIINA!). 171 



rest of the series barely twice as broad (see fig. 57). In Platurus, Emydocephalus, and 

 Aipysurus they attain their maximum development, being more than three times 

 as broad in the whole body length and very similar to the same shields in Colubrine 

 terrestrial snakes. 



. Numerically the value of these shields is of importance in distinguishing certain 

 genera, but in closely allied species like those of the genus Distira, the range of varia- 

 tion in individuals is so considerable, and the figures of the specific ranges overlap so 

 much, that the assistance to be derived from the number of these shields is decidedly 

 limited. They are fewest in the genera Emydocephalus , Aipysurus, and Hydrelaps, 

 being less than 200 ; most numerous in Distira fasciata where they may exceed 500. 



The specific range of variation depends largely upon the numerical strength of the 

 individuals available. If we exclude species but poorly represented numerically, the 

 smallest range of variation is that met with in Distira jerdoni (219 to 248), and D. 

 viperina (235 to 267) ; on the other hand the largest ranges of variation are seen in 

 Distira fasciata (376 to 531), D. torquata (310 to 438), andD. cyanocincta (280 to 397). 



The ventrals of most species are entire or mostly entire in the whole body length, 

 the few shields that are divided being seen about the umbilical scar and before the 

 anus. In Hydrus platurus, and in some specimens of Distira major, many of the shields 

 are divided, but subject to a good deal of variation in number and position in indi- 

 viduals. In D. cantoris and D. gracilis all the shields in the posterior half or so of the 

 body are very constantly divided (see fig. 13). In the very broad shields in some 

 Emydocephalus and Platurus a median obtuse keel is seen posteriorly, but this is an 

 inconstant feature found in only certain individuals, and, I believe, irrespective of age 

 and sex. 



The remarks made with reference to keels, tubercles, etc., under costals apply 

 equally to these shields. 



The ventrals in many of the species, specially in the genus Distira, are often very 

 difficult to count accurately. The difficulty may arise from the detail of these shields 

 being obscured by damage, desquamation, a local sodden condition, or the puckering in 

 places occasioned by the way the specimen has been folded in the bottle. Often too 

 small scales are interpolated on one or other side, which would alter the count on the 

 two sides. Some observers count these, some do not. Again some appear not to count 

 the shields which may be broken up, especially those just before the anus, and others 

 again do not count the early ill-developed ones in the neck. The result is that the 

 counts of various authors for the same specimen differ considerably. To take a single 

 instance, the type-specimen of Distira cyanocincta has 308 ventrals according to Russell, 

 296 according to Boulenger, more than 320 according to Giinther, and I count them 

 310. It is not very unusual for me to make these shields a little different in three or 

 four counts in the same specimen, which may appear extraordinary for one to confess 

 who strives at accuracy : still it is the fact. 



Costals. — (The ' ' scales " of other authors) . The importance of these shields in 

 classification is only second to that of the ventrals. 



