490 THE PALEONTOLOGY OF MINNESOTA. — 
(Lamellibranchiata. 
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. bellistriata Hall. 
It will be seen that the foregoing description of this 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. belli- 
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. bellistriata and A. 
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. bellistriata 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 other 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 2 
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 Vanuxemia, 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, Amphicelia, Hall, may 
