526. PRIMARY GROUPS OF MAMMALS. 
On TAE CHARACTERISTICS OF THE Primary GROUPS OF THE CLASS 
or MamMats. By Dr. THEODORE GILL. 
Ar the last meeting of the Association, the author made a com- 
munication on the classification of mammals, based on facts in 
part already become the common property of science, and in part 
hitherto unpublished. An abstract-giving the conclusions arrived 
at has been published in the American Narurarisr and in the 
Proceedings of the Association. Continued researches into the 
same subject have confirmed the propriety of the ordinal groups 
and the limits then admitted; but have necessitated a different com- 
bination of those groups. 
The divisions into sub-classes first solidly established by Hux- 
ley are retained. 
The Placental or Monodelphian mammals are with more propri- 
ety combinable into two major groups which correspond; on the 
one hand, to the Epucasri1a of Bonaparte, the combined ARCHEN- 
CEPHALA and GYRENCEPHALA of Owen, and the combined AR- 
cHonts and Mecasruenss of Dana; and on the other hand, to the 
Inepucasiria of Bonaparte, the MicrencerHata of Owen and the 
Microstuenes of Dana. The characters hitherto used to distin- 
guish those groups are however either vague and difficult of appli- 
cation, not characteristic, or generally regarded as erroneous. But 
positive and easily recognizable characters appear to exist in the 
brain which confirm those groups, but which have not hitherto 
been regarded, at least in respect to their systematic application. 
There has also'always existed cause to deplore the insufficiency 
of the characters assigned in the diagnoses of some of the orders 
of mammals. After an attentive study of most of the known 
forms, the author believes that he has succeeded in finding charac- 
ters which at the same time confirm the groups already recognized 
and supplement the teleological characters (sometimes of doubt- 
ful application or entirely failing) by morphological characters of 
more constancy. The revised diagnoses of the orders and -other 
primary divisions are submitted in advance of a work now being 
printed by the Smithsonian Institution; that work will give the 
characters, contrasted in dichotomous tables, of all the groups of 
mammals as low as subfamilies and lists of the genera, recent and 
extinct. While the author has been dependent, for the most part, 
