364: Eeport op the State Geolcgist. 



Parastrophia divergens. 



Plate 10, figs. 11-14. 



Parastrophia divergens, Hall. Palaeontology of N. Y., vol. VIII, pt. 2, p. 

 366, pi. Ixiii, figs. 4-7. 1894. 



Shell of medium size with strongly convex brachial valve and 

 depressed convex, anteriorly concave pedicle-valve. The beak of 

 the pedicle- valve is erect, but not conspicuous ; from the gently 

 convex umbo the surface slopes gradually to the lateral margins, 

 and abruptly to the front, forming a broad and. deep sinus, which 

 is sharply defined at the sides, and bears from two to four angu- 

 lar plications. Two or more smaller plications occur on each 

 lateral slope. The brachial valve is well rounded in the umbonal 

 region, but the median fold is defined only near the anterior 

 margin. It bears from three to ^nq plications, with three on 

 each lateral slope. All the plications, as well as fold and sinus, 

 become obsolete in the umbonal region, and in old and thickened 

 shells the latter can be distinguished only at the anterior margins 

 of the valves. In the interior there is a supported spondylium 

 in the pedicle-valve, but in the brachial valve the septal plates do 

 not unite. 



Hudson River group. Wilmington, Illinois. 



Parastrophia Greenii. 



Plate 10, figs. 1-5. 

 Parastrophia Greenii, Hall. Palaeontology of N. Y., vol. VIII, pt. 2, p. 367, 

 pi. Ixiii, figs. 17-20, 22. 1694. 



Shell robust, with convex brachial valve and shallow pedicle- 

 valve, convex in the umbonal region, but concave anteriorly. 

 Beak-s not prominent; that of the pedicle- valve low but erect; 

 that of the brachial valve full and incurved. Cardinal slopes 

 sharply defined on pedicle-valve. Median fold and sinus on 

 brachial and pedicle-valves not strongly defined except at the 

 anterior margin. The brachial valve bears six broadly rounded 

 plications which are obsolete in the umbonal region; four of 

 these belong to the median fold, the other two to the lateral 

 slopes. The pedicle-valve has five plications, with three in the 

 median sinus. Interior with a median supporting septum in each 

 valve. 



Niagara dolomites. JVear Milwaukee, Wisconsin. 



