49S THE PALEONTOLOGY OF MINNESOTA. 
{Byssonychia. 
The broad undulations of the surface distinguish the species from the other 
shells referred to Clionychia. 
Formation and locality.—Middle Galena, Fillmore and Goodhue counties, Minnesota: associated 
with Zygospira uphami W.and S., Vanuxemia abrupta Ulrich and Liehas ( Hoplolichas) robbinsi Ulrich. 
The original specimen is from the Trenton limestone at Watertown, New York. 
Genus BYSSONYCHIA, n. gen. 
Ambony chia (part.), HALL, 1847. Pal. New York, vol. i, p. 163. 
Ambonychia, HALL, 1859. Pal. New York, vol. iii, pp. 209 and 523; also of all American and 
European authors who have described that genus subsequent to 
this date. 
General aspect as in Ambonychia, Hall, excepting that the beaks and umbones 
are not so full. A well-defined byssal opening in the upper half of the anterior side. 
Hinge with a striated ligamental area, several small cardinal teeth and generally 
two or three slender lateral teeth near the posterior extremity. Posterior adductor 
impressions large, situated a little behind the center of the valves. Pallial line 
simple, terminating in the rostral cavity. 
Type: Ambonychia radiata Hall. (See fig. 55, v1, p. 477.) 
The erection of this genus became a necessity when a critical study of Ambony- 
chia bellistriata Hall, and several other species undoubtedly congeneric with that 
peculiar type of the genus Ambonychia, proved them to be without not only lateral 
teeth but a byssal opening as well. On the other hand Byssonychia has nothing like 
the anterior subrostral clavicle, while the external radiating cost are nearly always 
stronger than in Ambonychia. We have, therefore. at least three ordinarily valid 
generic differences to separate the two genera. Indeed, there is room for one or 
more intermediate genera. Two very nearly such groups actually exist in the Cin- 
cinnati rocks and | hope to publish descriptions of them in the next (7th) report-of 
the state geologist of Ohio. One (Allonychia) will contain, besides the type, Ambony- 
chia (Megambonia) jamesi Meek, two new species. They are all more erect shells, 
possessing a protruding byssal opening, a short hinge with wide ligamental area, 
but neither cardinal nor lateral teeth. The other group (Eridonychia) is based upon 
several elongate new species, having but little incurved beaks, scarcely ventricose 
umbones, a long and narrow byssal opening, thin hinge plate and no teeth. 
Byssonychia is closely related to Anomalodonta, Miller, but is distinguished by its 
hinge, that genus having neither true lateral nor cardinal teeth. It is to be admitted, 
however, that in certain species, otherwise precisely like Byssonychia, the posterior 
lateral teeth are nearly or quite obsolete. Descriptions of these and other new 
species of this genus have been written for the Ohio work above mentioned. 
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