68 CONTRIBUTIONS TO THE GEOLOGY OF MAINE. [bull. 165. 
length from the beak; mesial sinus deep in front, flat in the bottom, the side sloping 
outward, becoming obsolete at two-thirds the length of the shell; beak well defined, 
incurved down to the umbo of the dorsal valve; on each side of the beak a small 
ovate excavation extending toward the cardinal angles; umbo only slightly elevated, 
this valve being only half a line longer than the dorsal. Dorsal valve strongly and 
uniformly convex, nearly hemispherical, most elevated in the upper half, with a wide 
flat mesial fold elevated about one line above the general surface at the front margin, 
but becoming obsolete at two-thirds of the length; beak pointed, well defined, but 
only visible when that of the opposite valve is broken away. 
Surface with from twenty-five to thirty ribs, of which there are from four to six on 
mesial fold and from three to five in the sinus. All the ribs die out before reaching 
the beak, the upper one-fourth of the shell being smooth. On approaching the front 
margin the ribs are obscurely angular, but toward the upper part of the shell they 
are neatly rounded but not strongly elevated. 
Width, ten to twelve lines; length, about one-fifth or one-sixth less than width; 
depth of both valves, six to eight lines; depth of the sinus in the front margin of the 
dorsal valve, about three lines. 
The curvature of the ventral valve from the beak along the median line and the 
bottom of the sinus is a uniform arch somewhat most abruptly curved in the upper 
half. This curve can not be seen in the profile of the whole shell, as it lies partly in 
the sinus. 
This species is closely allied to R. abrupta Hall, Pal. N. Y., vol. 3, p. 228, pi. 32, 
fig. 3, but differs therefrom in being more ventricose in the upper part of the dorsal 
valve, in having the ventral valve uniformly arched along the sinus instead of 
abruptly bent upward nearly at right angles to the dorsal valve, and in not having 
the ribs in the sinus bifid at their extremities. 
Locality and formation. — Square Lake, Maine; Upper Silurian. 1 
Rhynchonella nucleolata Hall. 
R. nucleolata Hall, Pal. N. Y., vol. 3, p. 227, PI. XXI, figs. 1 and 2. PL III, fig. 5. 
Rhynchonella nucleolata; a, dorsal, b, front, and c, side views. 
Description. — This species is smaller than R. mainensis, and has the surface marked 
by only fifteen or twenty ribs toward the front, the upper part being smooth. The 
outline is subcircular or subpentagonal; the width varies from a little less to a little 
greater than the length. Ventral valve depressed convex, a small portion of the 
margin abruptly deflected to meet the margin of the dorsal valve; the mesial sinus 
becomes obsolete about the middle of the shell, but on approaching the front margin 
the portion of the shell constituting the bottom of the sinus is suddenly bent almost 
at right angles to the plane of the lateral margin and forms a projection which fits 
into a deep oblong notch of the opposite valve. Beak small, pointed, apparently 
incurved so as just to cover that of the dorsal valve. (It is not well preserved in the 
specimen examined.) Dorsal valve very convex, strongly elevated in the first half, 
where there is a short mesial fold; the upper half of the shell varies greatly in the 
amount of its gibbosity, being in some depressed convex, and in others rather 
strongly and broadly tumid; beak angular, prominent, but concealed beneath that 
of the ventral valve. There is a short mesial septum extending from the beak of 
this valve nearly half the length, which can be distinctly seen when the shell is par- 
tially or wholly removed. 
Of this species there are only two good specimens in the collection from Square 
Lake. It is very difficult to say whether they should be referred to R. nucleolata or 
to R. pyramidata. One of them has the dorsal valve strongly convex in the upper 
^roc. Portland Soc. Nat. Hist., Vol. 1, Pt. II, p. 110. 
