56 
CONTRIBUTIONS TO THE GEOLOGY OF MAINE. 
[bull. 165. 
50. Meristella arcuata. 
51. Strophostylus cf. elegans. 
52. Murchisonia hebe. 
53. Platyeeras, 8-10 distinct forms (? spe- 
cies). (=B31 and 32.) 
54. Gastropods, undetermined. (=?B33.) 
55. Orthoceras sp. ? (See B34.) 
56. Phacops trajanus. (=B36.) 
57. Proetus macrobius. (=B37.) 
58. Proetus Junius. (=B38.) 
59. Bronteus pompileus. (=B39. 
60. Lichas bigsbyi. (=B40.) 
61. Cypricardinia lamellosa. 
62. Lamellibranch, undetermined. 
The age of this limestone is distinctly the same as that of the Lower 
Helderberg formation of New York, as Billings has akeady stated. 
The precise correlation of the Square Lake fauna is to be determined 
by comparison of the two faunas, species by species. 
A glance at the lists shows us that there are many species peculiar 
to the Maine facies, and the question arises: Is it possible to recog- 
nize in the Maine fauna the stage of development corresponding to 
one or other of the stages exhibited by the rich subfaunas of the New 
York section? 
Although the full discussion of the biologic characteristics of these 
faunas is reserved for a future work, it may be of interest to state 
here the principles upon which correlation can be established on 
grounds of biologic evolution alone, and to illustrate them by a typi- 
cal discussion. 
The rhynchonellas of the Square Lake fauna are abundant and well 
preserved, and we haA^e in the several New York faunas of the Neosi- 
lurian and Eodevonian abundant representatives of this genus. There- 
fore the true rhynchonellas, and the forms which, although distributed 
in other genera, are evidently, from an evolutional point of view, 
closely related to them, have been selected. 
If fossils can be relied on to signify the stage of evolutional muta- 
tion in which they lived, a study of a single group should furnish 
direct evidence of the stage of the fauna investigated. Many figures 
have been made in the course of these investigations, and although the 
subject can be satisfactorily treated only in an elaborate illustrated 
monograph, the principles of the study and an indication of the 
importance and definiteness of the results obtained can be indicated, 
preliminary to the preparation of such a monograph, for which much 
minute study and a great number of illustrations will be required. 
CORRELATION OF THE RHYNCHONELLAS OF THE SQUARE LAKE FAUNA. 
GENERIC CHARACTERS. 
In estimating the geologic age of any fauna, the biologic relations of 
the species of each genus may be studied separately, and bear their 
own testimony as to the position they occupy in the general evolution 
of the race. 
