108 The American Geologist. Fobrnary, i897 
cation is (in the dorsal valve like <). viii/fiserJa Meek. (Jrlhis 
hifbvldu Sow., belongs, one might say, to the (). iiniltisectd 
type, even though its dorsal valve is more convex, and its hinge 
shorter even than O. fersa Sar. Regarding (>. eleganfula Ual., 
its internal characters are very like those of O. corjfulenta, etc., 
from which it differs in relative convexity of the valves, and 
in length, breadth, etc. (>. eltujautala Dal., if derived from any 
Ordovician "0. testadiiidvia,'''' has newly acquired a greater 
proportionate length of test, and the plication shows a corres- 
ponding modification. Small ventral valves have a single me- 
dian plication which does not remain, however, in the median 
plane, but one of the first offset plications becomes median; 
this, in turn, branches, and so on, in such a manner as to form 
a conspicuous branching right and left from the median line 
or plane. Several plications on this one are added to the sin- 
gle median plication of (K corpitleiifjt. Branching on the rest 
of the valve is, as a rule, from the side of the older plication 
in direction away from the median line, or posteriori}''. The 
external and internal characters in this species are such as 
might be considered to be those of O. eorptilenfa Sar., modified, 
by extension of the anterior shell margin. Orthis hi/brida 
Sow., being of the O. riiultisecfd t^'pe, and 0. elfu/ajituJa Dal., 
of nearly the O. corpulenta tj'pe, it would further seem in- 
consistant to adopt James Hall's view* and place O. kyhridd 
in a separate genus or sub-genus from -'O. testudinaria'''' and 
0. elegcDituIa, and particularly so if one is to consider O. m.eeki 
at the same time, simply as a ''coarse- ribbed varit^ty" of O. 
testu.dlnarid. O. variuxevii HuW, represented by a few speci- 
mens at hand, agrees with O. hi/brida in having a single medi- 
an plication on the dorsal instead of ventral valve, and the 
other species of that type, bear to O. vanitxenti a very close 
resemblance. 
The relation between Orfhis multisecta Meek, Orfhis cinac- 
erata Hall and the other above described O. testudin((ria auct.. 
may be made somewhat clearer by comparison to one or two 
other groups of that genus. Firstly, Ortltis (Platystrophia) 
biforafa Schl. appears in the Fucoid bed (5), and persists 
in succeeding beds 6, 7, 8, 9, 10, and again in 12, 13, and 14 
in the Galena and Maquoketa series. This is the same si:)ecies 
*Pa]. N. York, vol. viii, pt. 1, p. 
