72 CONTKIBUTIONS TO THE GEOLOGY OF MAINE. [bull. 165. 
left for remark at another occasion. The following deductions may 
be drawn : 
1. These several described species evidently belong- to a common 
race, whatever other species may belong to the same race, or whatever 
opinion we may have as to their generic relations. 
2. It is also clear that the dominant characteristics of R. mutabilis 
of the first stage are represented at the second stage by R. nucleolata, 
at the third stage by R. ventricosa, and at the fourth stage by R. spe- 
ciosa in part. 
4. Rkynehonella mutabilis of the first stage has the plications low, 
rounded, and not sharply developed; it shows various degrees of con- 
vexity, but marked gibbosity is the dominant characteristic; the fold 
and sinus are low and inconspicuous. The curvature of surface is 
gradual and even, so that nowhere does the surface present any marked 
or abrupt angles — i. e., any marked and sudden changes in the direction 
of growth of the shell. There is considerable variation expressed, 
but these characters prevail so dominantly among them all that there 
seem to be no characters by which to break up the series into separate 
specific groups. When the succeeding species are compared with this 
one, the differences are seen to consist chiefly in the intensifying of 
characters which are expressed here in a medium stage of development. 
4. There are evident at the second stage, the Delthyris Shaly lime- 
stone, three dominant forms (a, b, c) which are closely related to R. 
mutabilis, but differ by the intensifying of the central and the two 
extremes of character-combinations seen in that species. These are: 
a. The median type, which preserves the full complement of plica- 
tions, but they are more sharply defined upon the surface. The gib- 
bosity is distinctly less, though still conspicuous; the fold and sinus 
are well defined on the latter half of the growth of the shell, and the 
front is rather abruptly angulated, though not strongly so. This is 
called R. abrupta, and in the thinner forms R. vellicata. 
b. The extremely gibbous forms, which have a reduced number of 
plications that are fainter at the beak but become stronger with growth 
and more angular; the fold and sinus are well defined on the mature 
half of the shell. This is R. nucleolata. 
c. The form with least tendency to gibbosity, which grows directly 
from the beak, with flat surfaces, slightly arched plications, few and 
not evident at first, but well defined toward the front; the fold and 
sinus not sharply developed till late in the growth; and finally at the 
edge along the sides and at the front a sharp angular turn in growth, 
making a flat, almost square, front and sides. This is R. pyramidata. 
5. The third stage shows two dominant forms: 
a. The median type is further elaborated by an increased growth 
to the front, making an oval -elongated species with full complement of 
plications, well defined; the fold and sinus evident at front, and the 
