PLATE LXV. 
(Figures 1-3, 5 by G. B. Simpson ; 4, 6-9 by E. Emmons.) 
Genus CONCHIDIUM, Ltnne. 
Page 231. 
CoNCHiDiuM DECUSSATUM, Wliiteaves. 
Fig-. 1. Ventral view of a specimen partially exfoliated in the umbonal region; showing the fine, dupli¬ 
cate plication and the delicate concentric lineation of the surface. 
Fig. 2. Doi-sal view of the umbonal portion of an internal cast; showing the cavity of the spondylium 
and median sexitum of the xiedicle-valve, and the genital markings about the beak. 
Niagara group. Grrand, Rapids of the Saskatchewan River, British America. 
CONCHIDIUM LAQUEATUM, Coiirad (=Pentamekus NOBiLis, Emiiioiis). 
Fig. 3. An anterior view of a xiortion of the interior, looking into the chamber produced by the union of 
the sxiondylium and septal xilates ; showing the curvature of the latter, their explanate upper 
surfaces, and the foreshortened crural axioxihyses. 
Fig. .5. Another view of the same sxiecimen ; showing the siiondylium with a xiortion of the median sep¬ 
tum adhering, and the extent of the seiital x^lates and crural xirocesses. 
Figs. 4, 6, 9. Dorsal, ventral and iirofile views of an internal cast of a rather narrow shell; showing the 
contour, character of the xilication and smooth umbonal slopes. 
Fig. 7. Longitudinal section of conjoined valves ; showing the extent of the median sexitum of the pedicle- 
valve quite to the anterior margin of the valve, its concave anterioi- edge, the projecting ex¬ 
tremity of the spondylium, and in the brachial valve, the relatively short sxiondylium and 
sexita, and the xirojection of the crural axiox^hyses. 
Fig. 8. An internal cast of a brachial valve which retaines the iirevailing broader form of the sxiecies. 
Niagara dolomites. Delphi, Indiana. 
