LAMELLIBRANCHIATA. . 489 
Lamellibranchiata.] 
Class LAMELLIBRANCHIATA. 
( PELECYPODA.) 
Family AMBONYCHIID, Miller. 
Valves equal, very inequilateral; beaks prominent, terminal or nearly so; posterior 
cardinal region more or less alate; anterior side abruptly convex, with or without a 
byssal opening. Small cardinal and elongate posterior lateral teeth may be present 
or wanting. Posterior adductor impression large, bilobed (the upper part probably 
formed by a pedal muscle), situated above and behind the center of the valves. 
Anterior adductor wanting or very small, situated in the umbonal region. Pallial 
line simple, strongly impressed in the anterior region, becoming obsolete near the 
anterior extremity of the hinge. 
This family is unquestionably a valid one, and readily distinguished from the 
Aviculide with which its old genera are usually associated. In that family of shells 
the valves are always unequal and drawn out in front of the beaks into a distinct 
wing or lobe. The Ambonychiida, on the contrary, are always equivalved and with- 
out an anterior wing, the situation of the beaks being approximately terminal. 
As may be seen from the scheme of classification on page 485, I have extended 
the limits of the family so as to include several genera that are very differently 
arranged by other authors. Thus Amphicelia, Hall, is regarded as the type of a new 
family by Miller, while Whitfield has said that the genus is probably identical with 
Leptodomus, McCoy, and Meek and Worthen placed it near Pterinea. But, as I shall 
show in another work, Amphicwlia possesses every essential character of the present 
family. Palwocardia, likewise founded by Hall upon a Niagara species, also is closely 
related to Ambonychia. Hall’s Mytilarca and Plethomytilus again, can be shown, I 
believe, to be direct descendants of Lower Silurian types of this family and should 
not be placed with the Mytilide. 
Genus AMBONYCHIA, Hall (emend. Ulrich). 
Ambonychia (part.), HALL. 1847. Pal. N. Y., vol. i, p. 163. Not Ambonychia, Hali, 1859, Pal. N. 
Y., vol, iii, pp. 269 and 523; nor of American and European authors 
generally. 
Equivalved and profoundly inequilateral shells; valves ventricose, very thin, 
closing tightly all around; beaks full, strongly incurved. Surface with fine radiating 
strive, crossed by concentric growth lines and obscure undulations. Internally a thin 
plate passes vertically down from the anterior end of the hinge plate separating a 
