RHYNCHONELLA SPECIES OF SQUARE LAKE FAUNA. 
(>7 
may be defined as their more exact reproduction or transmission in 
generation. What distinguishes these Lower Helderberg rhy nchonellas 
as a whole is their great luxuriance and fertility, together with a great 
plasticity in the very characters which have been selected for both 
specific and generic diagnosis. This plasticity and increased range of 
variation, associated with the evidence of favorable conditions of life, 
does not point to restriction of the interbreeding which is supposed to 
distinguish specific groups, and it certainty does affect separate genera. 
With these remarks upon the generic characters, we may next 
examine the particular species present in the fauna and discuss their 
significance for purposes of correlation. 
SPECIES OP RHYNCHONELLIDS IN THE SQUARE LAKE FAUNA. 
(See Pis. I and II.) 
In the Square Lake fauna there are a considerable number of forms 
which fall at once into the Rhynchonella group of brachiopods, and which, 
upon a slight comparison with described species, are seen to be closely 
allied to species of Rhynchonella of the Lower Helderberg formation. 
In order to determine the geologic horizon of the fauna, not only is it 
necessary to determine the identity of these species, but, whether 
identical or not, it is necessary to ascertain the precise stage in the 
evolution of each phylum, or race, to which the particular species 
belong. This may be reached by ascertaining how these particular 
species are related morphologically to forms whose place in a typical 
geologic section, such as that of New York, is known. 
The following species of Rhynchonella have been recognized in the 
Square Lake fauna by Billings, and later by the writer: 
Rhynchonella mainensis Billings, M 19, 
M22. 
Rhynchonella nucleolata Hall, M 23. 
Rhynchonella pyramidata Hall, M 20. 
Rhynchonella pyramidata Hall var. a, 
M24. 
Rhynchonella vellicata Hall, M 45. 
Rhynchonella aspasia Billings, M 26. 
Rhynchonella bialveata Hall. 
Rhynchonella transversa Hall. 
There is no difficulty in recognizing, in the collection under investi- 
gation, specimens similar to those referred to by Billings in his original 
description of the fauna. Because of the rarity of the volume in which 
the species were originally described (the papers as well as the speci- 
mens having been burned in the fire which destroyed the Portland 
Society building in 1866), Billings's original description is here tran- 
scribed: 
Rhynchonella mainensis, n.sp. 
PL III, fig. 4, Rhynchonella ma'ntensis, n. sp. 
Description.— Transversely ovate, length about one-fifth or one-sixth less than the 
width. Ventral valve moderately convex, most elevated at about one-third the 
