VI 
PEEFACE. 
well-known and very common circumstance with regard to several groups of 
animals. In our present researches there exists a remarkable example of this 
fact in the genus Proctotretus, consisting, as is now known, of at least fourteen 
species, all inhabiting the western coast of South America. These facts, 
interesting as they are, have never been sufficiently investigated, although, it 
must be confessed, there are so many anomalies in relation to this subject, that 
we must despair of ever reducing the facts in question to any thing like fixed 
laws. 
The close approximation of the R uniform and Hyliform groups of the 
Anourous Amphibia is strikingly illustrated by several new forms obtained by 
Mr. Darwin, which are so perfectly osculant between the two families, that it is 
difficult to assign them a decided location. And the addition of some bufonine 
forms in the family Raniclce, as at present constituted, and on the other hand of 
some amongst the Bnfonidce, which are no less raniform in their general structure 
and habits, render it increasingly probable that the single character of the 
presence or absence of superior maxillary teeth, must be considered as in- 
sufficient to constitute alone a natural distinctive family character. There are 
several minor points bearing upon the natural arrangement of the Anourous 
Amphibians, which are illustrated by the characters of some of the species 
now first described, which will doubtless at some future time assist in the 
construction of a classification of these animals, bearing at least a nearer 
approximation to their natural arrangement than any that has hitherto been 
promulgated. 
The Ophidians have been placed in the hands of Mons. Bibron, who is 
at the present time engaged in completing his admirable history of Reptiles, 
by the publication of those volumes which are devoted to this order ; and 
it must be considered a fortunate circumstance that the delay wffiich has 
taken place in the appearance of that portion of his labours, has thus aftbrded the 
opportunity of embodying in so perfect a work, the numerous discoveries of 
Mr. Darwin in this particular department of Erpetology. 
T. B. 
Hornsey, Sept. 2, 1843. 
