Williams.] THE GENERA OF THE TCHYNCHONELLIDiE. 57 
The rhynchonellas are particularly interesting for this purpose, 
because in the typical New York sections of the Lower Helderberg, 
which at first glance seems most nearly related to this Square Lake 
fauna, this genus is represented by an abundance of individuals which 
have presented a wide divergence of form and size. By examination 
of the facts we may expect, therefore, to discover the place occupied 
by the Maine specimens, generic-ally as well as specifically, in the his- 
tory of the mutation of the race. 
The characters of the Rhynchonellidse as a family were notably dif- 
ferentiated "very early, and have been perpetuated up to the present 
without departure, at any time, from the early derived type,' 1 as Hail 
and Clarke remarked in 1894. 1 In the family twenty-four genera are 
recognized by Schuchert. 2 Schuchert's list is a revision of Hall and 
Clarke's classification of 1893-94. 3 
Of these twent} r -four genera, fourteen are represented by Paleozoic 
species, as follows: 
1. Rhynchonella, Fisher de Waldheim, 
1809. 
2. Rhynchotrema, Hall, 1860. 
3. Rhynchotreta, Hall, 1879. 
4. Camarotoechia, Hall and Clarke, 
1893. 
5. Leiorhynchus, Hall, 1860. 
6. Wilsonia, Kayser, 1871. 
7. Uncinulus, Bavle, 1878. 
8. Plypothyris, King, 1848 (not Phil- 
lips, 1841). 
9. Pugnax, Hall and Clarke, 1893. 
10. Eatonia, Hall, 1857. 
11. Cyclorhina, Hall and Clarke, 1893. 
12. Rhychopora, King, 1856. 
13. Terebratuloidea, Waagen, 1883. 
14. Orthorhynchula, Hall and Clarke, 
1893. 
Of these genera, the following are reported from rocks of the Lower 
Helderberg or Oriskany age in North America, viz: Rhynchonella, 
Khynchotrema,Camarotoechia,Wilsonia,Uncinulus, Eatonia. The genus 
Rhynchotreta contains no species of as late age as Lower Helder- 
berg. The genera Leiorhynchus, Hypothyris, Pugnax, Cyclorhina, 
Rhychopora, and Terebratuloidea, so far as present knowledge extends, 
have no species as early as the Oriskany. 
In the discussion of the correlation of the Square Lake fauna, which 
contains an Eatonia, we are thus restricted to six genera: Rhyncho- 
nella, Rhynchotrema, Camaroteechia, Wilsonia, Uncinulus, and Eatonia. 
Eatonia may be removed from the discussion for the time on account 
of its marked difference- from the other forms. It is important, first, 
to ascertain what are the generic characters of the other five groups, 
since the characters for determining the particular age must be sought 
among those which are subordinate to generic rank. 
Of these genera, Eatonia is represented in the Square Lake fauna 
by numerous specimens of Eatonia medialis. This species is also 
abundant in the Delthyris Shaly limestone of eastern New York. As 
' Pal. New York, Vol. VIII, Pt. II, p. 342. 
2 Bull. U. S. Geol. Survey No. 87, p. 121 . 
3 Kept. State geologist New York, Vol. T I. 1893, p. 935; and Pal. New York, Vol. VIII, Pt. II, 1894, p. 305 
