SYNOPSIS OF GENERA. Iv 



Surface marked by fine concentric striae with some faint curving radiating 

 striae on the body of the shell. 



Hinge marked by an elongate groove and fold. Ligament linear. Anterior 

 muscular impression just beneath the beak and occupying the upper part of the 

 distinct anterior lobe, which is separated from the body of the shell by a byssal 

 sinus. Posterior muscular impression large and shallow. Pallial line simple, 

 parallel to the margin of the shell. 



The shell is, in all respects, very similar to some recent forms of Modiola. 

 Type and example : Modiella pygmaa, pi. Ixxv, figs. 9-20. 



Megambonia, Hall (Palaeontology of New York, vol. iii, p. 273, and Twelfth 



Report of the State Cabinet of Natural History, p. 12. 1859). 



[Type, Megambonia cardiiformis. Hall.] 



Shell equivalve or sub-equivalve, very inequilateral, obovoid, body very 

 oblique ; anterior end (in two typical forms) lobed ; posterior large, constituting 

 the principal part of the shell ; beaks anterior ; cardinal line short, sub-alate 

 posteriorly ; umbonal slope ventricose, not defined. 



Surface marked by fine striae of growth, which in some species are crossed 

 by fine radiating striae. Hinge-line short ; a distinct lateral fold and groove 

 near the post-cardinal angle. Ligament internal. Anterior muscular impres- 

 sion large and strong ; posterior impression large. 



The type of this genus is well marked, and its form and external characters 

 are very distinctive. It does not, however, show the anterior teeth or crenula,- 

 tions mentioned in the original description. 



In the third volume of the Palaeontology of New York, several species are 

 described under this designation, which farther study has shown not to be con- 

 generic. At the present time the only forms which I regard as strictly belong- 

 ing to the genus are from the Corniferous limestone and Oriskany sandstone. 

 Those from the Lower Helderberg group require more material for a final 

 determination. 



Examples : Megambonia cardiiformis, pi. iii, figs. 1-8. 



Megambonia bellistriata, pi. cix, fig. 4 ; Pal. N. Y., vol. iii. 

 Megambonia ? lamellosa, pi. cix, figs. 5, 6 ; Pal. N. Y., vol. iii. 



