70 CONTRIBUTIONS TO THE GEOLOGY OF MAINE. [bull. 165. 
But, judging from the wide range of plasticity expressed in specimens 
of the Delthyris Shaly limestone, as well as in those of the Square Lake 
limestone, it is assumed that the limits of plasticity of this character 
were as great in the Delthyris Shaly limestone as in the Maine repre- 
sentatives, and that the difference in the description is chiefly sub- 
jective and depends upon the selection of type specimens made by 
Billings for his species R. mainensis. 
The second distinctive character noted by Billings is u the ventral 
valve uniformly arched along the sinus, instead of abruptly bent upward 
nearly at right angles to the dorsal valve." This is a characteristic of 
all the Square Lake representatives, except R. pyramidata} But, as 
between the R. mutabilis and R. abrupta, the Maine forms are decidedly 
nearer the mutation R. abrupta than R. mutabilis. 
The third character mentioned by Billings — the absence of the bifid 
extremities' of the plications — is not a constant character, as some of 
the Square Lake specimens are bifid at the extremities. 2 This character 
is associated with the abruptness of the bending of the surface at the 
front. In forms of the R. pyramidata type this character is con- 
spicuous; and, also, on a comparison of the successive mutations of 
the series, it is observed to be more conspicuous with advance of time. 
From such a comparison it seems probable that the angularity of the 
front, the sharp bifid development of the terminal part of the plica- 
tions, the general flattening of the shells during the first half of the 
growth, and the dominance of the fewer-ribbed specimens, are charac- 
ters associated with advancing evolution, and hence are more con- 
spicuous at the later stages of mutation. 
Passing from the characters of typical R. mainensis of Billings to 
closely allied forms associated together in the Square Lake fauna, the 
range of variation covers the same characters expressed by the varia- 
tions in the Delthyris Shaly limestone series. On the other hand, the 
variant with the high number of plications, flat form, and low sinus 
and fold, called R. vellicata by Hall, is well represented in the Square 
Lake fauna; 3 also the forms with fewer plications but gibbous form, 
the variant R. nucleolatq Hall, and those with few strong plications, 
angular beak, abrupt geniculated front, called R. pyramidata Hall, 
are represented together in the collection in hand. That these differ- 
ences are purely variational is well shown by two specimens in the 
Square Lake fauna. 
The first is a specimen of R. nucleolata (23/M), 4 which, on the young 
part of the shell about the beak, exhibits the full quota of plications, 
twenty -eight to thirty being clearly visible. Upon passing a prom- 
inent concentric line of growth in midst of shell the plications lessen, 
and at the front the number that can be counted is but eighteen, thus 
i See PI. II, figs. 12, 15, 18. 8 See PI. I, figs. 1-12. 
2 See PI. II, figs. 13, 15. 4 PI. II, fig. 21. 
