THE VOYAGE OF THE BEAGLE 109 



some other naturalists, makes this a sub-genus of the rattle- 

 snake, and intermediate between it and the viper. In con- 

 firmation of this opinion, I observed a fact, which appears 

 to me very curious and instructive, as showing how every 

 character, even though it may be in some degree independ- 

 ent of structure, has a tendency to vary by slow degrees. 

 The extremity of the tail of this snake is terminated by a 

 point, which is very slightly enlarged; and as the animal 

 glides along, it constantly vibrates the last inch; and this 

 part striking against the dry grass and brushwood, produces 

 a rattling noise, which can be distinctly heard at the dis- 

 tance of six feet. As often as the animal was irritated or 

 surprised, its tail was shaken; and the vibrations were ex- 

 tremely rapid. Even as long as the body retained its irrita- 

 bility, a tendency to this habitual movement was evident. 

 This Trigonocephalus has, therefore, in some respects the 

 structure of a viper, with the habits of a rattlesnake: the 

 noise, however, being produced by a simpler device. The 

 expression of this snake's face was hideous and fierce ; the 

 pupil consisted of a vertical slit in a mottled and coppery 

 iris; the jaws were broad at the base, and the nose termi- 

 nated in a triangular projection. I do not think I ever saw 

 anything more ugly, excepting, perhaps, some of the vam- 

 pire bats. I imagine this repulsive aspect originates from 

 the features being placed in positions, with respect to each 

 other, somewhat proportional to those of the human face; 

 and thus we obtain a scale of hideousness. 



Amongst the Batrachian reptiles, I found only one little 

 toad (Phryniscus nigricans), which was most singular from 

 its colour. If we imagine, first, that it had been steeped in 

 the blackest ink, and then, when dry, allowed to crawl over 

 a board, freshly painted with the brightest vermilion, so 

 as to colour the soles of its feet and parts of its stomach, a 

 good idea of its appearance will be gained. If it had been 

 an unnamed species, surely it ought to have been called 

 Diabolicus, for it is a fit toad to preach in the ear of Eve. 

 Instead of being nocturnal in its habits, as other toads are, 

 and living in damp obscure recesses, it crawls during the heat 

 of the day about the dry sand-hillocks and arid plains, where 

 not a single drop of water can be found. It must necessarily 



