50 CORRELATION OF GEOLOGICAL FAUNAS. [bull. 210. 
cies of each fauna as determined by study of the statistics; third, 
the general laws regarding the history of faunas and their use in 
interpreting the correlation of formations and the structure and devel- 
opment of the continent. 
The faunas specifically examined are: 
1. Fauna of the Hamilton formation, which may be called the 
Tropidoleptus carinalus fauna. 
2. Fauna of the Ithaca formation, which may be called the Pro- 
ductella speciosa fauna. 
3. Fauna of the Chemung formation, designated the Spirifer dis- 
junct us fauna. 
Other faunas and subfaunas will be named as they are taken up, but 
the statistics of these three faunas are ample and the3 T are of a like 
facies," so that their comparison will make evident the laws of shifting 
of faunas and their modification coincident with this shifting, with 
geographical distribution and with stratigraphical succession. 
HAMILTON FORMATION AND TROPIDOLEPTUS CARINATUS FAUNA. 
In the final report on the geology of the Fourth district of New York 
(1843) the Hamilton group was defined as the twenty-fourth group of 
the New York system, and with others was included in the Erie 
division. In the later classification, of which Dana's Manual of 
Geology, fourth edition, 1894, may stand as an exponent, the Hamil- 
ton group includes the Marcellus shale, the Goniatite beds, the 
Encrinal beds, the Hamilton shales, and the Moscow shales. The 
Tully limestone is also included by some authors; for the present 
discussion, however, this local formation may be treated faunJly as 
a separate formation. Faunally, the series of sediments, as tar / are 
exhibited in central New York (beginning at the top of the Ononda 
(Corniferous) limestone and terminating at the base of the Tully lime 
stone), presents a continuity which leaves no doubt as to the genetic 
succession of a common fauna from the base to the top. In dealing 
with this fauna, only the species between the limits of the top of the 
Onondaga limestone and the base of the Tully limestone, when these 
are present, will be considered as belonging typically to the Tropi- 
doleptus fauna. But the published lists are, on the one hand, too full, 
because they contain all the species which have been reported from 
the Hamilton group or formation, however that formation has been 
identified; and, on the other hand, they are not sufficient for the 
purposes of this paper, because locality and place in the formation 
are not always recorded or known. It has been necessary, therefore, 
to use specially prepared statistics. 
In order to ascertain the average characteristics of the fauna, a 
«In a paper read before the Geological Society of America after the present bulletin had gone 
to press I proposed the term homeotopic to express this likeness of facies of these faunas. See 
Bull. Geol. Soc. Am., Vol. XIV, 1903. 
