The Galena and Maqnoketa Series. — S((rde.soii. 91 
THE GALENA AND MAQUOKETA SERIES. 
By F. W. S.VRDESON, Minneapolis, Minnesota. 
(Plates IV and V.) 
Part III. 
On Okthis testudinaria. 
It has been claimed by Beecher and Clarke that the very 
small young shells of the genus Orthls are not distinguishable 
by any specific characters, "that the diagnostic characters of 
species of this genus may not be assumed until the earlier 
stages of the shell have passed."* It is further remarkable 
that the cited observation was made by these authors more par- 
ticularly concerning the shells of Orthis eleyantula Dalman and 
O. hybrida Sowerby, notwithstanding the fact that the shells 
of those species at 2.5 mm. or even at 0.5 mm, are easily distin- 
guishable by their plication alone, provided one does not mis- 
take the dorsal (brachial) valye of O. hybrida for the ventral 
(pedicle) valve. Plicated, bilaterally symmetrical shells like 
tliat of Orthis have one plication or interplication falling in 
the median plane in each valve, oftenest the one valve having 
a median plication, the other a median interplication, and 
small shells of Orthis hybrida Sowerby are distinguished from 
those of O. elegantula Dalman in that, by the former the dor- 
sal valve, by the latter the ventral valve only has a median 
plication. No one seems to have noticed that ditference until 
now. 
The distinction just mentioned came incidentally to my no- 
tice while engaged in tracing some of the progressive series of. 
variations in some Galena (Trenton) and Maquoketa (Hud- 
son) fossil species. It has been observed, too, that the de- 
scribed ditference between the two Silurian species Orthis 
[Rhipidomella) hybrida Sow. and 0. (Daimanella) eleyantula 
Dal. can be traced from an origin, apparently in the O. {Dai- 
manella) testudinaria •auqX., of the iipper Ordovieian system. 
The varieties of 0. testudinaria bear a very instructive relation 
to each other and to the two named Silurian species, and that 
relation described in connection with the geologic occurrence 
of each will, I think, be worth presenting in the following dis- 
cussion. 
*Beecher and Clarke, Memoirs of the New York State Museum, vol. 
1. No. 1, p. 13, 14, and 17. 
