The Galena and Maquokela Series. — Sardeson. 107 
them is not at hand. There are certainly other "O. testiidina- 
r/'a''' than the above described, even in America. 
Owing to the close morphological likeness between the spe- 
cies which have been above described, it seemed necessar)^ to 
assume that they must be phylologically near one another, 
and I have tried to explain how, among other characters, the 
differences in plication of the valves evolved. Some of the 
species have a single median plication on the ventral valve, 
like nearly all Orthis, others, on the contrary, have the single 
median plication on the dorsal, and a double median plication 
of the ventral valve. Probabl}^ the latter has been derived 
from the former, but the difference, however derived, is seen 
not only in the mature shells, but likewise in the smallest of 
plicated ones. The primary cause of the plication of Orthia 
I do not know, unless it is in the modification of the mesial 
circumference relative to the.convexity of the valves. 
Beecher and Clarke have studied, with special attention, the 
development and characters of Orthis eler/antida ,T)ii\., and 0. 
hybrida Sow., in their well known work on the Brachiopoda 
of the Waldron shales, — Niagara, Silurian. These two species 
are so near " O. festiulinaria'^ that with the usual latitude, 
they might even be included it it. The results of their work 
must, therefore, not be passed without special comment. They 
give us the result of their study of an abundant collection 
of Orthis from Waldron, that the tests of 0. hybrida and O. 
elegantula are not distinguishable in extremely early stages of 
growth, — 2 or 2.5'"'" length — and "that the diagnostic 
characters of species of this genus may not be assumed until 
the earlier stages of the shell have passed.''* From a few 
specimens at hand, it appears to me that the plicationf at least 
should serve as a distinctive mark on shells of O.S"^'^^ if it is 
possible to distinguish in them the ventral valve of (>. hybrida 
Sow., from the dorsal of 0. elegantula Dal. The small — 2""™ 
— shells of the latter are like those of O. rogata and 0. corpu- 
lenta of the same size, in having a single median plication on 
the ventral valve. Those of O. hybrida Sow., as appears on little 
larger shells, have, from the tirst, a double median plication, i. 
e., a median furrow on the ventral and the single median pli- 
*See Memoirs of the Ne\v York State Museum, vol. i, no. 1, p. 13, 14 
and 17. 
tThose from Waldron and some from England are just alike. 
