74 Trans. Acad. Sci. of St. Louis. 



derhook, Illinois. The second form occurs in the Chonopec- 

 tus sandstone, and apparently presents constant characters 

 by which it may be separated from the shell in the limestone 

 above. It is always larger and thicker than the typical P. 

 striatocostata , and as they diverge from the beak the lateral 

 margins form a wider angle than in the limestone specimens. 

 The differences are so great and apparently so constant that 

 it is possible that the Chonopectus sandstone form should be 

 given a specific designation, but for the present it will be con- 

 sidered only as a variety. In volume eight of the New York 

 Paleontology this shell is illustrated on plate 62, figs. 44-45, 

 under the name P. missouriensis, while the typical form of 

 P. striatocostata is illustrated under the same name on plate 

 60, figs. 33-34. Allied species have been described from the 

 Waverly sandstone in Pennsylvania. 



Rhynchonella sp. undet. 



PI. II. f. 4-5. 



Several specimens of Rhynchonella have been observed in 

 the fauna of the Chonopectus sandstone, but they are too im- 

 perfect for certain identification. They seem to belong to a 

 single variable species, the two figures given representing the 

 extremes, one with an obtuse and the other with an acute 

 beak. The umbonal region of the pedicle valve is convex, 

 the sinus being present only towards the anterior margin of 

 the shell, and there it is very shallow. The anterior margin 

 of the valve is abruptly incurved so that the margin lies nearly 

 at a right angle to the general plane of the shell. The surface 

 of each valve is marked by about twelve or fourteen simple 

 rounded plications. Most of the specimens observed are 

 pedicle valves, only a few fragments of the brachial valve 

 having been seen. 



EUMETRIA ALTIROSTRIS (White). 



Pl.II.f. 18-19. 



Shell longitudinally subovate in outline, the valves nearly 

 equally convex. Brachial valve with a prominent, elevated 

 beak slightly incurved at the tip, and perforated by a large 

 circular foramen ; mesial sinus shallow, ill-defined. Brachial 



