LAMELLIBEANCHIATA. 489 



Lamellib ranchlata.! 



Class LAMELLIBRi^iNCHIATA. 



V (Pblecypoda.) 

 Family AMBONYCHIIDiE, 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 

 Aviculidm 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 Ambonychiidce, 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, 1 have, extended 

 the limits of the family so as to include several genera that are very differently 

 arranged by other authors. Thus Amjphiccelia, 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, Amphicalia possesses every essential character of the present 

 family. Palceocardia, likewise founded by Hall upon a Niagara species, also is closely 

 related to Amhonychia. 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 Mytilidce. 



Genus AMBONYCHIA, Hall (emend. Ulrich). 



Ambonyehia {p?iTt.), Hall. 1847. Pal. N. Y., vol. i, p. 163. Not Ambonyehia, Hall, 1859, Pal. N. 



Y., Tol, lii, 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 

 striae, 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 



