492 THE PALEONTOLOGY OF MINNESOTA. 
[Ambonychia bellistriata. 
AMBONYCHIA BELLISTRIATA /Hall. 
5 PLATE XXXY, FIGS. 1 and 2. 
Ambonychia bellistriata HALL, 1847. Pal. N. Y., vol. i, p. 163. Not Ambonychia bellistriata 8. A. 
Miller, 1874, Cin. Quart. Jour. Sci., vol. i, p. 14. 
The Minnesota specimen illustrated on the accompanying plates differs slightly 
in its outline from the original figures of the species given by Hall im the work 
cited.* The hinge line is a trifle longer and the anterior side less uniformly curved. 
Still, I cannot for a moment doubt its specific identity with the types of the species, 
since it possesses all the more essential characters. The beaks and umbones are 
very prominent and strongly incurved, and the radiating striz fine (about twelve or 
thirteen in 5 mm.) and apparently of the same character as in A. planistriata, except- 
ing that they show no traces of the fine concentric lines noticed in that species. 
Compared with A. planistriata the present species is found to differ in the relative 
narrowness and greater prominence of its umbones, and in wanting the shallow con- 
centric undulations, which are always a striking feature of that species. A. orbicu- 
laris is a more erect and rounded form, and not so ventricose. 
The name Ambonychia bellistriata occurs in all the published catalogues of the 
fossils of the Cincinnati group, but the species referred to in the lists is really a very 
different one. Indeed, it is a true member of the proposed genus Byssonychia, and 
closely related to the type of that genus, B. radiata Hall, sp. 
Formation and locality.—In the central part of the Trenton limestone at Middleville and Trenton 
Falls, New York; and in the middle Galena near Wykoff, Minnesota. 
AMBONYCHIA AFFINIS, 2. SD. 
PLATE XXXV, FIGS. 5—%. 
This species or variety is most probably a later phase of A. planistriata Hall, 
and as it resembles that species very greatly it will be sufficiently characterized by 
pointing out the differences. Thus, the beaks and umbones are a little less tumid 
and the convexity of the shell correspondingly less. The shell is also a trifle more 
erect and rounder, the hinge line slightly shorter and the postero-cardinal margin 
more rounded. Finally, the concentric undulations are much more obscure, while 
the radiating striae are coarser, there being only eight in 5 mm. to twelve in the 
same space for that species. At first I thought the species might prove the same as 
A. orbicularis Emmons, sp., but a comparison with Hall’s figures in vol. i of the Palz- 
ontology of New York, will show that the anterior side of the New York species is 
*An examination of the types of the species, which are now preserved in the American Museum of New York City, 
proves that figs. 4a and 4b (on plate 36) are faulty in showing the radiating lines stronger than natural. Indeed, they 
ure quite as strong in these figures as in the magnified views of the surface represented in fig. 4d. 
