VARIATIONS OP^ GARTER-SNAKES. 21 



However, since, as I shall show later, the males are smaller, 

 more elongate and less stocky in form than the females, a tendency 

 toward a reduction in the number of scale rows should, it seems to 

 me, be revealed first in the former sex. This point is difficult to test, 

 for extensive series are wanting in most regions where such a reduc- 

 tion is inaugurated, and in a region where the reduction is great 

 enough to markedly influence the females tlie difference between the 

 sexes would be obscured, since there is not enough difference in 

 relative size to give the males the next lower formula. A series of 

 specimens of radix from Palo Alto and Clay counties, Iowa, however 

 (where, as will be shown later, there is a slight tendency toward a 

 reduction in the number of scale rows from 21-19-17 to 19-21-19-17), 

 seem to confirm our reasoning, for out of seventeen males thirteen 

 have the formula 19-21-19-17, as against 21-19-17 in the others, 

 while out of thirty-five females but six have this formula, the re- 

 mainder' having 21-19-17. This matter should receive further 

 study. 



The law of variation to which the scale rows are subject may be 

 formulated as follows: The individual, geograjikic, and racial varia- 

 tions in the numher of dorsal scale rows in the garter-snalces is brought 

 about by the shortening and loss of the same scale rows as are ordinarily 

 drofped posteriorly in conformity with the taper of the body, and there 

 is evidence that this decrease is due to a dwarf ng of the body. 



It should be pointed out that the adjustment of the scale rows to the 

 taper of the body shown in these snakes is very similar to the plate 

 arrangement observed in certain Palseechinoids by Jackson and Jagger 

 (1896). If our diagram (fig. 1) be compared with their' diagram of the 

 ideal arrangement of interambulacral plates in Melonites multiporus 

 (p. 164) it will be seen that the rows dropped posteriorly in the snakes 

 are almost exactly in the middle (of each side), as the rows of inter- 

 ambulacral plates which are discontinued toward the ventral end in the 

 echinoids are in the middle of the interambulacral area, so that the 

 order in which they are dropped in the snakes and echini is exactly 

 the same. In the echinoid figured, owing to the pronounced con- 

 striction dorsally, the columns of plates are distorted, but continue 

 to be represented — a feature that is not shown in those garter-snakes 

 in which the maximum number of rows continues to the head, possibly 

 because the constriction of the neck is too slight to disturb the rows. 

 That a dying out of columns also occurs in echinoids, however, is 

 stated by Jackson and Jagger, and it is very interesting to note that 

 "when columns die out or cease to be continued to the dorsal area 

 it is commonly the middle or last added column which drops out 

 first in the cases observed," which is exactly analogous to the dropping 

 of the scale rows anteriorly in the garter-snakes. 

 33553— Bull. 61—08 3 



