PREFACE. XXVII 
Viviparous Quadrupeds of North America,’’ was prepared; the materials at their command were 
far less extensive than have been at mine, and that many species which they could only examine 
in the museum of London, Paris, Berlin, and Leyden, are now to be found in the Smithsonian 
collection in a profusion of specimens of the most satisfactory and perfect character. 
An apology is necessary for the delay which has taken place in the completion of the general 
reports on the zoology of the Pacific Railroad surveying parties. This has arisen from the fact 
that, from the first organization of these expeditions, in the spring of 1853, nearly to the present 
time, one or more has been in the field, and engaged in fresh examinations ; so that until all 
the specimens expected were received, the general systematic account of zoological results could 
not conveniently be prepared. The examination of the materials was actually commenced early 
in 1855, and many of the articles written in that year and in 1856. With the continuous 
accession of additional specimens, it became finally necessary to re-write, alter, or extend all 
that had been prepared prior to the present year (1857.) It is to this that the frequent want 
of uniformity is due, the time allowed not being sufficient in many cases to permit the re-working 
of the whole matter. The measurements of the specimens were at first made in inches and 
lines, but that of hundredths was finally adopted ; it is to this fact that the presence of the two 
different divisions of the inch is attributable, it not being convenient or possible to make the 
measurements conform throughout, as would have been desirable. 
It is, perhaps, unnecessary to state that the matter of the present report is entirely original 
throughout, the few cases in which extracts from other authors are made being so indicated. 
With very few exceptions, all the references in the synonymy have also been personally made 
and verified. Where this was not possible, the synonym is enclosed between quotation marks. 
In explanation of the too frequent occurrence of typographical errata in the body of this 
report, it is proper to state that, owing to various circumstances, the work was necessarily passed 
through the press with a rapidity probably unexampled in the history of natural history 
printing, allowing very little opportunity for that critical and leisurely examination so neces- 
sary in correcting a work of the kind. For most of the time the proof has been furnished and 
read at the rate of twenty-four to thirty-two pages per day ; nearly 400 pages having been set 
up, read, and printed during the first half of July alone. The same cause has also interfered 
with the preservation of perfect uniformity of arrangement and detail throughout. In some 
cases, accidents to the form while on the press have caused the loss or transposition of letters, 
words, or paragraphs; as, among others, the exchange of characters of Orders VIII and IX, 
on page 1, referred also to on page 625. For excuse of errors in the use of technical terms, in 
the formation and inflection of scientific names, and for all other shortcomings, the writer can 
only throw himself upon the kind indulgence of his readers, partly in consideration of the fact 
that, owing to the urgent necessity for a speedy completion of the volume, no time was allowed 
for any revision of the manuscript as a complete work, nor, indeed, of its separate portions, 
and that for much of the time the preparation of the manuscript was only a few hours in 
advance of its delivery to the compositor. 
A few words in explanation of the plan adopted in preparing the articles of the present 
report may not be out of place. I have usually made the entire detailed description of the 
species from one particular specimen, (often indicating it by number,) mentioning afterwards 
the variations presented from this type by the others before me. The specific diagnoses alone 
contain a combination or selection from the characters of several specimens. 
The numbers attached to the specimens, as enumerated, are those which they bear in the 
