498 THE PALEONTOLOGY OF MINNESOTA. 



[Bjraaonyohia. 



The broad undulations of the surface distinguish the species from the other 

 shells referred to Clionychia. 



Formation and locality. Middle Galena, Fillinorc and Goodhue counties, Minnesota: associated 

 with Zygospira uphami W. and S., Vanuxemia abrupta Ulrich and Lichas (Hoplolichas) robbinsi Ulrich. 

 The original specimen is from the Trenton limestone at Watertown, New York. 



Genus BYSSONYCHIA, n. gen. 



Ambonychia (part.), HALL, 1847. Pal. New York, vol. i, p. 163. 



Ambonychia, HALL, 1859. Pal. New York, vol. iij, 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: Ambonyckia radiata Hall. (See fig. 35, vi, 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 costae 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 I hope to publish descriptions of them in the next (7th) report of 

 the state geologist of Ohio. One(Allonychi) will contain, besides the type, Ambony- 

 fltin (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 (Kridonychia) 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. 



liystionychia 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. 



