INVERTEBRATES. 253 



The specimen figured by Prof. McChesney, under the name Rhynchonella 

 Parvini, seems not to have been in a condition to show very satisfactorily the 

 characters of the species, being merely disunited valves, though his description 

 agrees exactly with the specimens before us, which fact, taken in connection 

 with the locality and position, leaves little room for doubt in regard to its 

 identity with our shell. "We may also add, that Prof. McChesney, on seeing 

 our specimens, identified them with his R. Parvini. 



Locality and position: Keokuk, Iowa; Nauvoo and Warsaw, Illinois, etc.; 

 Keokuk division of the Subcarboniferous series. 



Genus CHONETES, Fischer, 1837. 



(Oryctogr. Mosc, p. 134.) 



Chonetes planumbona, M. and W. 



PI. 18, figs, la, lb, Ic, Id. 



Chonetes planumbona, Meek and Worthbn, Oct., 1860. Proceedings Acad. Nat. Sci., 

 Philad., p. 450. 



Shell of medium size, nearly semicircular ; length from two- 

 thirds to three-fourths the breadth ; hinge line usually a little 

 longer than the greatest breadth of the valves at any point 

 farther forward ; front rounded, or forming, with the sides, a 

 nearly semicircular curve ; lateral margins intersecting the 

 hinge at generally less than a right angle. Ventral valve 

 usually moderately gibbous, flattened at the umbo, most convex 

 near the middle, and sloping down to the sides and front — 

 destitute of any traces of a mesial sinus ; ears a little com- 

 pressed ; cardinal margin sloping very slightly from the beak 

 to the extremities, armed with some eight or ten oblique spines 

 (only known from small remaining bases) ; area of moderate 

 breadth ; foramen rather broad-triangular, the upper angle 

 being a little rounded ; beak not incurved. Dorsal valve con- 

 cave, or following nearly the curve of the other valve; cardinal 

 process moderately prominent, nearly or quite closing the 

 foramen of the opposite valve ; interior without a prominent 



