No. 39—1889.] ZOOLOGICAL TABLES. 



31 



alterations in the proportions of the shields in this little group 

 (the old family Oligodontidce), although there is no structural 

 difference. 



In No. 33, Helicops scJiistosus, there is only a single 

 internasal. This strikes the attention at once, and is a strong 

 structural character deserving undoubtedly strong expression ; 

 but the question is, are the proportional changes in the head 

 shields of the Oligodons of equal value ? Whilst some minds 

 would be more inclined to dwell on the points of likeness 

 between Simotes and Oligodon and Zamenis, others would be 

 more inclined to dwell on the peculiarities that characterise 

 the head shields of the two former genera. There is also the 

 subject of convenience to be considered, which depends much 

 on the formulae employed. These formulae should be as 

 simple as possible. Complicated formulae, such as aa\, aa\, 

 ab{, ab\, are neither easy to compare, to read, or to remem- 

 ber. In the following table the formulae consist of a single 

 letter and an index figure. 



For many years I have attempted to make the letters 

 expressions of absolute types that should at once appeal to 

 the eye and also agree in their structural characters, and 

 hoped that the index figures would serve to complete the 

 descriptions of the sub-divisions, so that all parts may be 

 referred to by the same letter and figure ; that is to say, all head 

 shields under a shall be really exactly alike. But there is no 

 likeness in nature. Not only do individuals differ, and the 

 same individual at different ages, but the same individual in 

 itself. In many snakes, especially in sea-snakes, the shields 

 differ on the two sides of the head. 



Moreover, different authorities do not take the same views. 

 The characters used by Dr. Gunther in the " Snakes of India " 

 are not those used by Dr. Boulenger in his "Reptiles of 

 India and Ceylon." 



The letters and figures therefore merely serve to combine 

 characters together that I trust may prove useful to the 

 student: the letters the more important characters and the 

 figures the minor characters. 



