WILLIAMS. 
billings's species' of rhynchonella. 69 
half, and the other depressed convex, but they both unquestionably belong to the 
same species. 
Locality and formation. — Square Lake, Maine; Upper Silurian. 1 
Rhynchonella aspasia. 
PL III, fig. 6, Rhynchonella aspasia. Dorsal and side views of a young individual. 
The large specimens have the base not so distinctly rectangular, but a little rounded. 
Description. — Transversely subelliptical ; greatest width about one-fourth more than 
the length. Ventral valve narrowly convex on the umbo; a mesial sinus commenc- 
ing at about one-fourth of the length from the beak, and growing deeper with an 
uniform curve to the front margin, where it is full one-half the whole width of the 
shell and deeply indents the opposite valve; on each side of the sinus one or two 
ribs more prominent than the others; outside of these the surface is depressed con- 
vex, and shows a tendency to become slightly concave close to the margin; beak 
small, closely incurved and nearly, if not quite, in contact with the umbo of the 
opposite valve. Dorsal valve rather strongly but narrowly convex along the median 
line; sloping to the sides; mesial fold strongly elevated in front, dying'out at about 
half the length. Surface in young specimens with seven or eight strong angular ribs 
on each valve; one rib in the sinus and two on the mesial fold. In old specimens, 
about twelve ribs on each valve, of which there are two or three in the sinus and 
three or four on the fold. 
The greatest width is about the middle of the shell. On a side view of a young 
shell the most elevated point of the dorsal is just at the front margin on the top of 
the fold, and the base has a rectangular form. In the old specimens, on a side view, 
the front is not so decidedly angular, but slightly rounded. Width of the largest 
specimen, 7 lines; length, 6 lines. Width of the smallest specimen, 4 J lines; length, 
3i lines. 
This species is allied to both E. altiplicata and E. acutiplicata, Hall, Pal. N. Y., 
vol. 3, PI. XXXIII, and seems to stand between them. It is proportionally wider 
than the former and not so numerously ribbed as the latter. It may be that a 
greater number of specimens would afford us the means of uniting all three. 
Locality and formation. — Square Lake, Maine; Upper Silurian. 2 
The typical Rhynchonella mamensis Billings 3 is plainly a representa- 
tive of R. abrupta Hall of the Delthyris Shaly limestone of New York. 
The first distinctive character noted by Billings is the greater ventri- 
cosity in the upper part of the dorsal valve of the Maine species. This 
is a character belonging to the earlier form R. mutabilis. At the Del- 
thyris Shaly limestone stage of mutation, of the forms with the higher 
number of plications, R. abrupta is the extreme in this character of 
early gibbosity. The extreme of gibbosity at this stage pertains to 
the forms with fewer plications, as R. nucleolata. This variation is less 
extreme in R. vellicata, and among the few specimens in the present 
collection there are those passing to R. vellicata, and even thinner than 
the typical R. vellicata of New York. 4 
If the modification were associated purely with geologic range 
it would be inferred that the Maine fauna was younger than the R. 
mutabilis stage in New York, and older than the R. abrupta stage. 
i Proc. Portland Soc. Nat. Hist., Vol. I, Pt. II, p. 110. 3 See PI. II, figs. 11, 14,17. 
2 Op. cit., Vol. I, Pt. II, p. 111. 4 See PI. I, figs. 1, 5, 9. 
