\() Hans Gadow, 



witli or without yellow collar. 2. Brown with a black bar across the 

 nape, and with black spots or bars on the back. 3. Heacl and nape 

 black, separated by a yellow collar; body red or redbrown, uniform 

 or spotted, or banded, with black. 



It is interesting- to note that in South Eastern Mexico occurs 

 also the closely allied species St. diadetnatus, which is uniform dark 

 brown or black above, with the exception of the yellow band across 

 the head. I found such black specimens within a few hundred yards 

 of St. atratus, both under stones, but on perhaps uuder more rocky 

 and open ground. It is further suggestive that near Orizaba, and 

 thence to the Isthmus of Tehuantepec occurs also Homalocranium 

 rubrum, the usual colour of which is quite red and spotless like 

 specimen I of Sf. atratus. 



The monochrome red dress represents the terminal, not the 

 incipient stage of the whole series. 



Comparison of the two series, and general cou- 

 clusions: In the DM. series the pale xanthic groundcolour is 

 encroached upon by the red and black patch-colours, and the black 

 may ultimately dominate the red, producing an almost monochrome 

 black snake. 



In the SE. series the ground colour is encroached upon by the 

 red patch- colour, and by the black which grows within the pale 

 interstitial bands; and then this black is eventually driven out by 

 the red patch-colour. The ultimate possibility is the production of 

 an almost monochrome red snake. 



In both series therefore those colours will dominate which 

 originated in the patches, probably because these patches represent 

 growing points, directing centres of metabolic activity. Originally 

 these centres were most like?y quite segmental, but this metamerism 

 has been lost long ago through confluence of neighbouring spots 

 into larger, more effective units, and in the majority of our snakes 

 they arrange themselves in transverse bars. The type of procedure 

 characteristic of Lizards: longitudinal stripes breaking up into spots, 

 rearrangement into crossbars, and eventually ultimate monochromes 

 — does not apply to these snakes. With them erythrism means 

 suppression of the black, and melanism means the. suppression of 

 the red pigment, in both cases by conceutric Invasion. The yellow^ 

 is a more stabile lipochrome than orange or red. Since the latter is 

 so easily destroyed by light, we may assume that this red owes its 

 existence toadeflciencyofcertain environmental light; a condition which 



