DORSAL SCALE- ROWS TN BRITISH SNAKES. 



359 



19-17. 



The drojp from 19 to 17 rows takes place by the fusion of 

 series TV* with III. (see text-fig. 2) or occasionally with V. (the 

 Vlths of the original 21). The final loss of this row nsnally takes 

 place at a point 3/5ths down the body, although the complete 

 range of variation is from '55 to •75. 



Text-fiofure 2. 



1K+Iz= 



Cornnella aiistriaca, lateral view, to show fusion of series IV. and III., 

 '^tlis down the body. 



The fusion of series IV. Avith III. (or V.) is in active progress, 

 for one finds («) individuals on which the reduction from 19 to 

 17 rows takes place at a definite point on the third quarter of 

 the body, anterior to which the number is invariably 19, and 

 [b) individuals on which series IV. fuses, or tends to fuse, inter- 

 ruptedly with III. on the entire anterior half of the body, and 

 continues in this way until that point is reached, on tlie third 

 (juarter of the bodj^ where the loss is final. Between these two 

 extremes every intermediate form may be found, the degree even 

 varying on the two sides of the bod}'. This instability shows a 

 tendency for this loss to become complete. 

 17-15. 



The reduction from 17 to 15 rows takes place by the suppression 

 of series III. or its fusion with IV. (the Vlth of the original 21), 

 the final loss being at a point on the posterior quarter of the body, 

 but subject within this limit to considei'able variation, which 

 suggests that this reduction is compai'atively recent, and still in 

 active progress. 



3. Vij^era herus.- — The apparently most primitive form among 

 living Vipers, F". ursinii, also has 19 rows of scales, and there- 

 fore V. herus, with scale formulae varying from 19-17—16 to 

 23-21-19-17-16, must have four of its sibridged series in course 

 of production, aud three in course of reduction. 

 23-21. 



In the average individual there are 23 rows of scales imme- 



* Although in many cases the fusion is completed in such a manner as to make it 

 impossible to say which is the supernumerary row, I judge it to be IV., as in some 

 cases tlie final scales of this row are rudimentary, and also the occasional union with 

 the Vth instead of the IlIrd points to this conclusion. 



