494 THE PALEONTOLOGY OF MINNESOTA. 



[Ambonychla amygdallna. 



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 striae 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 aphcea, two 

 species described by flail from the Niagara rocks of Wisconsin and Illinois that 

 should go with Mytilarca and not with Clionychia. 



Clionyohia lamellosa Hall. 



PLATE XXXV, FIGS. 10-14. 



Ambonychia lamellosa Hall, 1861. Rept. Sup't. Geol. Sur. Wis., p. 31; Whitfield, 1882, Geol. 



Eep. Wis., vol. iv, p. 205. 

 Ambonychia attenuata Hall, 1861. Kep. Sup't. Geol. Sur. Wis., p. 33; Whitfield, 1882, Geol. 



Rep, Wis., Yol. iv, p. 206. 



Shell obliquely subquadrangular or subovate in outline; hinge 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 



