100 The American Geologist. Fibmary, i897 
The ventral valve has a double instead of single median i>licati()n, 
and an analysis into six pairsof fascicles of plications as in O. multiHevia 
is found to be constant. The number of plications in the median pair 
is variable, and often one of thesj is the extreme of its mate in this 
character, as in the specimen figured (fig. 3, \A. V). A slight converg- 
ence of the plications near the anterior margin is common. 
From O. corpnlenta with which this form has been confu3ed, the 
double instead of single median plication on the ventral valve, the finer 
plication, the more quadrangular outline, and the large-sized muscle 
scars, make a good distinction. The last character separates this from 
O. miiltisecta and O. meelci but in other respects it generally resembles 
either the one or the other of them. It is the successor to O. multinecta 
in the Maquoketa series, but it is contemporaneous in part with that 
one because the same did not become extinct elsewhere. With O. meeki 
S. A, Miller it certainly lived contemporaneously, as far as known. Its 
origin might be from earlier representatives of O. meeki, slight dwarf- 
ing having produced finer plication and convergence of the same, with 
shortening of the mesial periphery, esp::^cially at the anterior of the 
valves. 
ORTHIS TERSA Sardeson. 
PI. V. fig. 8 to 13. 
Orthis terisa Sardeson (1892) Bull. Minnesota Academy Nat. Sci. vol. 
Ill, p. 331, pi. 5, fig. 11, 12, 13. 
This species is found at Wilmington, Illinois, and at Nye (Iron 
Ridge) Wisconsin, in the horizon of the top of the Maquoketa series, 
and is therefore either contemporaneous or a close successor to O. meeki, 
O. multisecta and O. ignota. 
Orthis tersn has thickened valves as compared to other species, and 
the hinge is short, being only one-half the shell width. The ventral is 
strongly convex in front of the beak with an increased median convex- 
ity which soon flattens and is replaced by a broad median sinus. The 
dorsal valve is convex and has a narrow median depression that disap- 
jjears at the margin of some mature shells. The anterior margin is 
therefore straightened or inflected, and this same with the gradual 
sloping of the margin from the cardinal angles gives a triangular ten- 
dency to the transversely oval outline. The beak on the ventral valve 
is strongly curved, that on the dorsal is also curved although it is short, 
and hence the beaks are directly opposed and not far apart. The car- 
dinal areas are small since the hinge is short. The foramen, too, is 
small, but the teeth and sockets, the cardinal process and the crura are 
large, while the septum tapers forward. The muscle scars are nearly 
intermediate to those of O. ignota and O. multisecta, but the ventral 
scars diverge more than in either of them. 
The outer surfaces at the beaks have been eroded before fossilization 
and probably during life so that even the initial points of the cardinal 
process and the crura are bared. Partial removal of the outer shell is 
not peculiar, but the excessive wear in this case led to inquiry whether 
the characters of O. tei'sa were not due to erosion of the cardinal angles 
