NIAGARA LIMESTONE OF CHICAGO, ILLINOIS. 07 



Edmondia de Koninck. 

 Edmondia Nilesi W. and M. 



Plate II. figure 13. 



Shell rather small, equivalve, transverse, once and a half as long as wide, subventricose 

 with a prominent umbonal ridge extending to the postero-ventral angle, becoming some- 

 what acute near the beak, and leaning over toward the hinge, so as to cause the subter- 

 minal beaks to project considerably beyond the slightly arched hinge-line and to have their 

 apices twisted forward. The general outline is oval, a little widening posteriorly, and flat- 

 tened along the dorsal side as far as the middle. Beneath the beaks is an anterior lunette, 

 forming a small notch in the outline. Ventral side gently curved. Shell thin; its exterior 

 nearly smooth, and marked by faint incremental lines. The casts do not enable us to deter- 

 mine the character of the muscular impressions. Greatest width, a little posterior to the 

 middle; greatest thickness, a little anterior to it. In the casts the beak is less terminal 

 than in the shell. 



Length, .95 inch; greatest width, .60 inch; greatest thickness of both valves, .40 inch. 



We have some hesitancy in making a generic reference of this species, thinking it may 

 be an Orthonota ; but the apparently edentulous hinge, and the slightly convex dorsal and 

 ventral sides seem to point to Edmondia. 



Dedicated to W. H. Niles of Cambridge, Mass., the discoverer of the species. 



Ambont/chia myiiloidea Hall. Wis. Geol. Rep. 1860, p. 2. Casts show concentric folds near 

 the margin. The shell was apparently thick. Resembles Myalina mytiliforntis Hall, (Pal. 

 N. Y., 100, PI. xxx. fig. 1,) from the Gray sandstone of the Clinton group. 



Conocardium Bronn. 

 Conocardium niagarense W. and M. 

 Plate II. figure 14. 



Shell small, ventricose, about twice as long as wide, with central beaks and finely costate 

 exterior. The truncation, or boundary of the ribbed portion is posterior to the middle of the 

 shell, and forms, on the posterior side, an angle of 58° with the straight, elongate dorsal 

 side. The form of this end resembles the outside of a salver. The rostrum is produced 

 into a linear extension reaching as far behind the beak as the opposite extremity is in front 

 of it. The beak is incurved, but not perceptibly turned one way or the other. The body of 

 the shell is ornamented by eighteen sharp and delicate ribs, the anterior one forming an 

 angle of 28° with the hingedine, and the included space being apparently smooth. 



Length of shell, about .46 inch; height, .26 inch. 



This shell is sometimes found with the two valves united. It is the only species of Cono- 

 cardium known to us in the Niagara group. 



It will be perceived that we denominate the rostral extremity the posterior one, contrary 

 to the usage of Pictet, McCoy, and others, but, as we believe, more in accordance with anal- 

 ogies. 



MEMOIKS BOST. SOC. NAT. HIST. Vol. I. 25 



