'1:1.1.11 KAN. Ill V |8I 



UiiiivlllbranrhUla.l 



Class LAMKUJl;i;\N< III \T.\. 



i l.K< \ II H)A.) 



K.unily AMBONYl llim.K, Miller. 



Valves i-qnal. very inequilateral: l>eaks prominent, terminal or nearly so; posterior 

 cat-.linal region more or less alate; anterior side abruptly convex, with or without a 

 byssal opening. Small cunlinal and elongate jM^t.-rior lateral teeth may be present 

 MI wanting. Posterior adductor impression large, bilobed (the upper part probably 

 fi.nin-1 liy a pedal muscle), situated above and behind the center of the valves. 

 Anterior adductor wanting or very small, situated in the umbonal region. Pallia! 

 line simple, strongly impressed in the anterior region, becoming obsolete near tin- 

 anterior extremity of the hinge. 



This family is unquestionably a valid one, and readily distinguished from the 



iiliiln 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 Ambonychiidtr, 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 Amphica-lia, Hall, is regarded as the type of a new 

 family by Miller, while Whitfield has said that the genus is probably identical with 

 Lcptodomus, McCoy, and Meek and Worthen placed it near Pteritua. But, as I shall 

 show in another work, .1 //<///>,/ Ha possesses every essential character of the present 

 family. Palteocur<li<i, likewise founded by Hall upon a Niagara species, also is closely 

 related to Ambonychin. Hall's Mytilarcn 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 Mylilitln . 



Genus AMBONYCHI A. Hull (emend. Ulrich). 



Ambonychuilp&Tl.), HALL. 1847. Pal. N > I, p. 13. Not AmbongcHia, Hal!. IV,y. Pal. M. 



Y., vol. Ill, pp. 200 and 5t3: nor of American aod European author* 

 generally. 



K.imvalvtl and profoundly inequilateral shells; valves ventricose, very thin, 

 dosing tightly all around; beaks full, strongly incurved. Surface with tine radia' 

 -tri ;e, crossed by concentric growth lines and obscure undulation-. Internally a thin 

 plate passes vertically down from the anterior end of tin- hin^e plate separating a 



