» 
38 PROCEEDINGS OF THE NATIONAL MUSEUM. VOL. XXXIV. 
Second. In the case of geographic distribution, at the same geo- 
logical horizon, we are restricted to heredity and environmental influ- 
ences. Modifications of form, therefore, which are expressed only 
as between faunules of separate localities are accounted for as occa- 
sioned primarily by the differing conditions of environment of the 
separate localities. 
Third. Finally, in the case of modifications of form observed in - 
the faunules of successive stratigraphic stages of the same locality, 
the distinguishing differences, so far as they are other than those of 
the local faunule and other than those of the geographic provinces, 
are evolutional. 
It would not be safe to conclude that in producing any one of 
these three classes of modification only one of the assigned forces has 
been in operation, but only that in each case the one indicated has ~ 
been chiefly operative. 
The Dalmanellas of the Chemung formation of New York State 
are closely enough related to each other morphologically to make it 
probable that they are all of common descent. The question of their 
relationship to Dalmanellas of earlier geologic periods, or to those of 
other geographic provinces of the Devonian, can be determined with 
greater precision after the facts of their relations to each other have 
been thoroughly established. We may also leave for later con- 
sideration the question whether the subgroups into which they are 
divisible do or do not show indication of intersterility. As a whole 
they constitute for the time and place a well-defined group of organ. 
isms which for the present may be regarded as a genus. The sub- 
groups into which the genus is divided will be called species. 
THE GHNERIC CHARACTERS OF DALMANELLA. 
The name Dalmanella was proposed by Hall and Clark in 18924 
for a subdivision of the genus Orthis Dalman. This group of Orthis 
had been previously called “ group of Orthis testudinaria Dalman ” 
by Hall in the year 1890.° 
The characters of Orthis testudinaria perpetuated in the Devonian 
Dalmanellas are as follows: 
First. The general form varies in outline from subcircular ¢ to sub- 
quadrate @ and wide-hinged forms.° | 
Second. The pedicle valve has a small but elevated beak from which 
a blunt carination of the outer surface extends, broadening and flat- 
tening toward the front. 3 
Third. The brachial valve has a sulcus along the center which 
broadens toward the front. 
CERI, ING Wo WIUUL, Ieee, jo, AO, 
6 Bull. Geol. Soe. of Am., I, pp. 19-205. 
Gel, ING Yoo, WIG, Ieee, jo 20, fol, Wie, ies, ZL 
d Wigs. 27 and 33. 
ms: 9 
7 ries, SY 
a 
