18 



BULLETIN 61, UNITED STATES NATIONAL MUSEUM. 



The decrease in the number of scale rows posteriorly is, we believe, 



correlated with the taper of 

 the body, for as the scales 

 themselves-decrease but little 

 in size toward the tail the loss 

 of the rows is a necessary re- 

 sult of the shortening of the 

 metameres. Furthermore, 

 as shown above, the reduc- 

 tion posteriorly takes place 

 caudad to the middle of the 

 body, i. e., where the body 

 tapers to the tail. This phe- 

 nomenon may not in itself 

 be considered important, but, 

 as will be shown directly, it 

 has a wider significance than 

 as an explanation of the man- 

 ner in which the scutellation 

 accommodates itself to the 

 posterior diminution in the 

 size of the body. 



In certain forms the maxi- 

 mum number of scale rows is 

 known to be very constant. 

 For example, specimens of 

 sirtalis, eques, proximus, sau- 

 ritus, or sacJceni are but very 

 rarely found that have either 

 more or less than 1 9 rows an- 

 teriorly, but, notwithstand- 

 ing the fact that most of the 

 other forms are also usually 

 described as having a definite 

 number of scale rows, there 

 is considerable variation in 

 this respect. If a specimen 

 of megalops with a maximum 

 number of 23 rows be exam- 

 ined it will be found that this 

 number only occurs on the 

 middle of the body, owing to 

 the fact that the fifth row is 

 dropped anteriorly as well as 

 posteriorly, thus making a formula of 21-23-21-19-17, counting the 



