460 THE PALEONTOLOGY OF MINNESOTA. 



[Khynchot re in a ina'i(ui valviv. 



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. capax, except that in the present 

 species it is very shallow, owing to the shells not being thickened as in E. 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 four 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. minnesotensis. 



*l':in:t<li!in Nul. iind Gool., vol. iv,-p. 443, 18511. 



