490 THE PALEONTOLOGY OF MINNESOTA. 



[Lamelllbranohiata . 



small lobe, immediately beneath and sometimes a little in front of the beaks, from the 

 umbonal cavity. Hinge plate narrow, with a few ligament striations and two small 

 oblique cardinal teeth; no lateral teeth. Muscular impressions and pallial line very 

 faint. 



Type: A. hellistriata Hall. 



It will be seen that the foregoing description ofthis genus is, in many respects, 

 widely different from that adopted by all preceding authors. Hall's original diag- 

 nosis is, of course, too broad and on the whole indefinite, since it included species 

 which subsequent study proved to be quite different from the typical species. Again, 

 the commonly accepted characterization of Ambonychia, since the publication of 

 Hall's notes On the genus in 1859, is based upon his A. radiata and not upon A. helli- 

 striata, which, of all the species placed under Ambonychia by him in 1847, alone is 

 entitled to the distinction of being the type. Ambonychia, therefore, as generally 

 understood, is synonymous with the group of shells which now propose to name 

 Byssonychia, and quite distinct from Ambonychia as based upon A. hellistriata and vl. 

 orbicularis (Emmons), the two species first following the original description of the 

 genus. 



This new interpretation of the genus may produce some confusion, but it is 

 necessitated by the rule of priority, which demands that when no type is mentioned 

 the first species to- follow the original description must be regarded as the type of 

 the genus. Having then no alternative but to accept A. hellistriata as the type, I 

 have redefined the genus in accordance with the characters presented by that species 

 and four others, A. orbicularis Emmons, A. planistriata Hall, A. affinis, n. sp., and A. 

 amygdalina Hall, all of which, with the possible exception of the last, are unques- 

 tionably congeneric. 



Compared with xjther members of the family, Ambonychia, as here understood, 

 differs from Clionychia, Ulrich, in having a small lobe-like cavity beneath the beaks 

 where, in that genus, there is a mere thickening of the margin of the valves. In 

 casts of the interior the whole upper part of the anterior side of Clionychia is 

 impressed to the edge of the valves, while in Ambonychia the same part presents a 

 small protruding, vertically elongate lobe, separated from the anterior side of the 

 rostral cavity by a sharply-impressed thin line. This lobe reminds one greatly of 

 the anterior adductor impression of Vdnuxemia, but I could not satisfy myself that 

 it really lodged such a muscle. Other differences are that in Ambonychia the valves 

 are more ventricose and the umbones and beaks more strongly incurved, while the 

 surface is marked not only concentrically but also radially. In Byssonychia there is 

 a byssal opening in the anterior side and the hinge is strengthened by two or three 

 slender posterior lateral teeth. The Upper Silurian genus, Amphicoelia, Hall, may 



