58 CONTRIBUTIONS TO THE GEOLOGY OF MAINE. [bull. 165. 
the genus is not reported at a lower stage than the Lower Helderberg, 1 
and the species is not known from a lower horizon than the Delthyris 
Shaly limestone, the presumption is strong, from this evidence alone, 
that the Maine fauna is not much older than the Delthyris Shaly lime- 
stone of the New York section. 
Of the family Rhynchonellidae, the following genera are recognized 
in America at lower horizons than the Lower Helderberg: 
Orthorhynchula Ordovician. 
Rhynchotrema Ordovician. 
Rhynchotreta Silurian. 
Camarotcechia Ordovician-Silurian. 
Wilsonia Silurian. 
Uncinulus Silurian. 
Rhynchonella Silurian. 
Thus it is shown that the ancestral origin of the Lower Helderberg 
forms must be sought within these seven genera. 
The only species of Uncinulus reported from below the Lower 
Helderberg is TJ. strichiandi, from Waldron, Indiana, Louisville, Ken- 
tuck} 7 , and strata of Niagara age. Rhynchotrema is abundantly repre- 
sented in the Ordovician by B. capax, reported from Indiana, Ohio, 
Illinois, Missouri, Wisconsin, Iowa, Minnesota, Anticosti, Manitoba, 
and Hudson Bay. The closely allied species B. inwquivalve is reported 
from the Trenton of Drummonds Island, New York; Kentucky; Ten- 
nessee; Illinois; Wisconsin; Iowa; Minnesota; New Mexico; Ottawa, 
Canada; Manitoba; and a few other species are reported; but no species 
of this genus is recorded from the lower horizons of the Silurian. The 
Lower Helderberg form B. formosa is the latest representative of 
the genus recorded for America. The genus is thus characteristically 
Ordovician in age, and the propriety of referring the Lower Helder- 
berg species to the genus calls for careful examination. 
The fact that the small specimens referred by Hall 2 to BhyncJio- 
trema formosa in the Delthyris Shaly limestone of New York can not 
be distinguished from specimens of the Square Lake fauna which are 
here referred to Bhynchonella aspasia, makes it probable that rhyn- 
chonellas of this type may be the direct descendants of the rhyncho- 
tremas of the earlier formations. More study, however, will be 
necessary to confirm this view. 
By this method of exclusion we are restricted to the species cata- 
logued under the names Rhynchonella, Orthorhynchula, Rhynchotreta, 
Camarotcechia, Wilsonia, and Uncinulus for the probable ancestors of 
the rhynchonellas of our fauna. The most characteristic rhyncho- 
nellas of the New York Lower Helderberg are referred to the group 
called Uncinulus by Bayle. 3 
1 In the present paper this genus is reported from the limestone of Castle Hill, 1415 L, in association 
with species which signify Niagara age. (See p. 54.) 
2 Pal. New York, Vol. Ill, pi. 35, fig. 3. 
8 See Hall and Clarke, 1891 and 1893, and Schuchert, 1897. 
