12 INDEX GENERUM MAMMALIUM. 
tioned in the first description was adopted instead. Later on the types 
fixed by subsequent authors and revisers of groups were noted by 
inserting the word ‘type’ in parenthesis after the species so indicated 
by the first reviser, and by marking the reference to the paper from 
which this information was obtained ‘type fixed.’ All this of course 
necessitated a reexamination of many volumes and greatly delayed the 
progress of the work. Some cases which should have been reexamined 
may have been overlooked, thus adding another possible source of error. 
These details are mentioned, not to magnify the difficulties of the work 
or to condone errors which it may contain, but merely to show the 
probability of finding mistakes in an index of this kind in spite of the 
checks adopted to detect them. 
Although nearly twenty years have elapsed since this index was first 
projected, very little headway was made until 1891, and the work has 
been actually in progress only about twelve years. The long delay in 
bringing it to completion has been due largely tothe desultory way in 
which the work had to be done, chiefly at odd moments in the intervals 
between more important official duties. Changes in the plan and the 
reexamination of references delayed it far more than would otherwise 
have been the case. Slow progress in undertakings of this kind is, how- 
ever, not unusual, as shown by Bronn's elaborate * Index Paleontolo- 
gicus,’ which was fifteen years in course of preparation. The present 
index was supposed to have been almost ready for publication in 1894, 
but had it been issued then it would have comprised only the alpha- 
betical index of genera (Part I) and only 80 per cent of the names now 
included. "The delay has resulted in enlarging the original scope of the 
work, the addition of nearly 1,000 names and much of the matter on 
etymology, and the incorporation of many corrections, which, although 
not perceptible, are none the less important. A number of rare books 
containing new names have been acquired, and several valuable general 
works recently published have been examined to the great benefit of 
the work. Among these may be mentioned Trouessart’s * Catalogus 
Mammalium, Roger’s * Verzeichniss der Fossilen Saügethiere,' Miller 
& Rehn’s * List of North American Land Mammals,’ Thomas's ‘Genera 
of Rodents,’ Sclater & Thomas’ * Book of Antelopes,^ W. L. Sclater’s 
* Mammals of South Africa, Lydekkers ‘Deer’ and *Oxen, Sheep, 
and Goats,’ the volumes on monkeys, marsupials, and British mammals 
in Allen's Naturalists’ Library, Beddard's ‘Mammals,’ Hay’s 'Cata- 
logue of Fossil Vertebrates of North America,’ Sherborn's ‘ Index 
Animalium," C. O. Waterhouse’s * Index Zoologicus, and numerous 
special monographs, including the paleontological papers of Ame- 
gbino, Hatcher, Matthew, Osborn, Roth, Scott, and Wortman. 
