494 THE PALEONTOLOGY OF MINNESOTA. 
[Ambonychia amygdalina. 
This well marked genus embraces probably the simplest and earliest types of 
the family, from which all the other genera descended. Yet, while a direct line to 
Mytilarca and Plethomytilus seems obvious enough, I must confess my inability to 
bridge over the gap between the radially ribbed genera on the one hand—and these 
form a very natural and closely interrelated group—and those in which the surface 
is marked with concentric lines only, on the other. At present, therefore, the evi- 
dence favors the conclusion that in times preceding the Chazy there existed a more 
primitive type still that combined the characters of the two groups. . 
Compared with Ambonychia, as here restricted, the present genus differs in its 
smaller umbones and less incurved beaks, in wanting radiating striz and in the 
structure of the anterior side, there being, instead of a clavicle-like plate or ridge 
beneath the beaks, a mere thickening of the margin, leaving a cavity or impression 
in the cast where that genus presents a small lobe. Mytilarca, Hall, which probably 
was not evolved till after the close of the Lower Silurian, is distinguished by its 
cardinal and posterior lateral teeth, and more oblique form. 
In the remarks following the original description of the genus I mentioned 
Ambonychia amygdalina Hall, as belonging here. This I now believe to have been 
an error. Respecting A. nitida and superba, described by Billings from Anticosti, 
and other concentrically marked species that have been referred to Ambonychia, it 
may suffice to say that they are not congeneric with the types of that genus. Their 
true relations cannot be established until we know something definite about their 
hinges. Some of the species in question are much like A. acutirostra and aphca, two 
species described by Hall from the Niagara rocks of Wisconsin and Illinois that 
should go with Mytilarca and not with Clionychia. 
CLioNycHIA LAMELLOSA Hall. 
PLATE XXXV, FIGS. 10—14. 
Ambonychia lamellosa HALL, 1861. Rept. Sup’t. Geol. Sur. Wis., p. 31; Wurrrrenp, 1882, Geol. 
Rep. Wis., vol. iv, p. 205. 
Ambonychia attenuata HALL, 1861. Rep. Sup’t. Geol. Sur. Wis., p. 38; WHITFIELD, 1882, Geol. 
Rep, Wis., vol. iv, p. 206. 
Shell obliquely subquadrangular or subovate in outline; binge line straight, 
generally but little shorter than the length of the shell beneath; anterior margin 
nearly straight, sloping backward five to fifteen degrees from a vertical line, below 
curving rather rapidly into the strongly convex basal line; posterior margin more 
gently curved, joining the hinge line sometimes sharply at other times gradually. 
Valves rather strongly convex, most ventricose in the umbonal region and near the 
anterior side where the slope to the edge is abrupt; cardinal slope gentle, in some 
cases nearly flat, in others distinctly concave. Beaks terminal, small, acutely 
