636 GEOLOGY OF OHIO. 



regions, compressed in the postero-cardinalpart; beaks moderately prom- 

 inent, incurved; byssal opening small, not impressed. Surface of each 

 valve with twenty to twenty-two more or less curved, strong, radiating 

 costse. In the best preserved specimen these are ridge-shaped and sepa- 

 rated by wide concave interspaces, while the whole surface is marked with 

 rather regular, sharp, concentric lines distinctly visible to the naked eye. 

 Ligamental area very narrow and short, hinge plate thin, posterior lateral 

 teeth wanting; one oblique cardinal tooth and socket in each valve. 



The hinge in this species is peculiar, having no lateral teeth and 

 only one cardinal tooth. It must be a transitional form and, as I believe, 

 from Byssonychia to the Eridonychia and Allonychia types of structure. 

 I cannot now enter into a discussion of these possible relationships, yet, 

 to avoid misunderstanding, I should say that I do not regard Eridonyichia 

 as having been evolved from B. byrnesi : another aberrant species, B. 

 actitirostris, being as it seems to me, better qualified to stand as a progen- 

 itor of that restricted genus. With Allonychia, however, the case proba- 

 bly is different, at any rate we have no conclusive evidence against the 

 view that this genus is genetically related to the species under considera- 

 tion. 



Considered as a species, B. (?) byrnesi is so easily recognized that com- 

 parisons with other forms are quite unnecessary. The specific name is 

 given in remembrance of the late Dr. R. M. Byrnes, of Cincinnati, Ohio. 



Formation and locality: Lower beds (Utica horizon) of the Cincin- 

 nati group, at Covington, Kentucky, where it occurs in the banks of the 

 Ohio and Licking rivers; also in an equivalent position at Nashville, 

 Tennessee. 



Genus ANOMALODONTA, S. A. Miller. 



Anomalodonta, S. A. Miller, 1874, Cin. Quart. Jour. Sci., Vol. I, p. 16; also 1889, 

 North Amer. Geol. aud Pal., p. 462. 



Shell suberect, equivalve, profoundly inequilateral, alate posteriori}-, 

 abrupt anteriorly; beaks terminal, incurved, umbones prominent; byssal 

 opening present, sharply defined. Surface with coarse, radiating costae. 

 Ligamental area broad, grooved longitudinally; beneath the posterior 

 half of the external area a variable number of large, irregular folds 

 which probably served for the attachment of an internal ligament; at 

 the anterior end of the hinge an oblique, irregularly pitted fold together 

 with a corresponding depression in one or both valves; the pits continue 

 downward as grooves to the byssal opening. Posterior adductor impres- 

 sion large, its inner margin deeply sinuate, situated in the middle of the 

 shell or more or less above and behind the center, the position varying 

 in the species according to the length of the shell; pallial line distinct, 

 extending forward from the posterior side of the large muscular scar in 

 a direction nearly parallel with the margin of the shell, bending around 



