713 



wise recorded by Lucas ('86, p. 562), and illustrating, according to 

 the terminology of Bateson, forward homoeosis. 



Of the four possible combinations that the two observed positions 

 of the sacrum and the first haemal arch give, it is interesting to ob- 

 serve that one — that having the sacrum on the twentieth vertebra 

 and the first haemal arch on the twenty-second — was never met with, 

 A sacrum on the twentieth vertebra and a first haemal arch on the 

 twenty-third was seen six times; a sacrum on the nineteenth and a 

 first haemal arch on the twenty-second, four times; and a sacrum on 

 the nineteenth and a first haemal arch on the twenty-third, fifteen 

 times. This last condition, from the frequency of its occurrence, is 

 obviously the typical one for the species. 



The two cases of unsymmetrical sacra can best be considered here. 

 In the first specimen (Fig. 1) the last trunk vertebra was number 

 eighteen and carried on its transverse processes greatly reduced ribs. 

 The sacrum was composed of vertebrae nineteen and twenty. The 

 nineteenth vertebra possessed on its left side a well developed sacral 

 rib to which the ilium was attached and on its right side a rudimen- 

 tary rib, like those on the eighteenth vertebra. The twentieth vertebra 

 was the reverse of the nineteenth, in that it carried on its right side 

 a sacral rib and on its left a 



Fig. 1. 



Pig. 2. 



rudimentary one. As is typical 

 with this species, vertebrae twenty- 

 one and twenty - two were without 

 ribs and the first haemal arch was 

 on vertebra twenty-three. The 

 conditions presented here are in 

 all essential respects those of a 

 typical sacrum except that the 

 right sacral rib, instead of being 

 attached to the nineteenth vertebra, 

 is connected with the twentieth, 

 a condition that might be designa- 

 ted as forward homoeosis. 



In the second specimen (Fig. 2) 

 the essential structural features 



Fig. 1. Ventral view of the unsymmetrical sacral region from the first specimen 

 of Necturus. The eighteenth and twenty-third vertebrae are indicated by numbers. 

 R, right; L, left. 



Fig. 2. Ventral view of the unsymmetrical sacral region from the second specimen 

 of Necturus. The nineteenth and twenty-fourth vertebrae are indicated by numbers. 

 iJ, right; L, left. 



47 



