460 THE PALEONTOLOGY OF MINNESOTA. 
LKhyiiebotrema inajquivalvis. 
the sinus; commonly, however, the number is seventeen or eighteen, four and three, 
respectively; in Kentucky specimens the tendency is to have fewer plications, there 
being from thirteen to eighteen on each valve, all crossed by exceedingly delicate 
concentric zigzag lines, sometimes subimbricating and conspicuous over the anterior 
half of the shell. 
Ventral valve strongly convex in the umbonal region and nearly flat on each 
side of the deep mesial sinus, sloping more or less abruptly laterally and often 
angular near the anterior margin; mesial sinus originating on the umbo, often 
profound anteriorly, with abrupt sides. Beak more or less incurved and always 
elevated beyond the umbo of the dorsal valve, with a narrow delthyrium partially 
closed by deltidial plates, which grow out from the walls of the former and, as far 
as observed, do not join medially. Hinge teeth prominent and supported by thin, 
short, dental plates. Muscular area much as in R. eapax, except that in the present 
species it is very shallow, owing to the shells not being thickened as in R. capax. 
Dorsal valve more convex than the other, with a mesial fold more or less 
strongly elevated anteriorly, beginning at the apex of the shell as a slight depression. 
Beak projecting into the delthyrium of the ventral valve. Crural plates large, sepa- 
rated medially by a depression which is partly occupied by a linear cardinal process 
strongly curved inward and upward, converging proximally and joining the angular 
median septum, which terminates at about the center of the valve on the crest of a 
plication; at the base of the crural plates and separated by the septum are two pairs 
of adductor scars, the anterior ones being the larger; dental sockets deep, situated 
lateral to the crural plates. 
The variations in this species are numerous, yet are never such as to be of value 
to the geologist, with one exception, which is described below as variety minnesotensis. 
R. incequivalvis has often been considered to merge into R. capax; this, however, is 
not known to occur anywhere in the Trenton formation, nor do the specimens of the 
Hudson River group, in their younger stages, look exactly like adult R. incequivalvis. 
The figures given of the two species readily show the differences between them. 
In the middle beds of the Trenton limestone a number of free and very well 
preserved specimens, collected by Mr. Ulrich, have from twelve to fourteen plications 
on each valve, and are globose and smaller than is usual for this species. Similar 
but larger shells also occur rarely in the shales above. These specimens approach 
Rhynchonella orientalis of Billings,* from the Chazy group, but differ from it in having 
three or foar more plications, and none of them has the straight, lateral outline 
shown in the second series of his figures. This is probably the form to which Mr. 
Sardeson has given the name R. tninnesotensis. 
♦Canadian Nat. and Geol., vol. iv, p. 443, 1859. 
