PELECYPODA—PRIONODESMACEA. 



411 



Differs from Ambonychia in the strong anterior muscular impres- 

 sion and the numerous teeth in the anterior portion. Sil uric— Car- 

 bonic. 



126. M. lata Hall. Devonic. 

 Anterior slope oblique ; posterior basal margin expanded. 



Gibbous in the middle and in the umbonal region. 

 Helderbergian : New York, Oklahoma. 



127. M.? aviculoidea Hall. (Fig. 531.) 



Upper Siluric and Lower Devonic. 



Surface marked with fine concentric striae 

 which are unequally lamellose. This may 

 be a Pterinea. 



Distinguished from similar species of 

 other genera by the very small anterior ear, 

 the strong anterior muscle scar characteristic 

 of the genus, and the posterior wing, not de- 

 fined from the rest of the shell by sinus or 

 change of ornamentation. 



Manlius : New York, New Jersey. Helderbergian: New York. 



128. M. suborbicularis Hall. (Fig. 532.) Devonic. 

 Anterior muscular impression prominent on internal molds. 



Type of the genus. 



New Scotland : New York. 



Fig. 531. Megambo* 

 nia ? aviculoidea, left 

 valve. (Pal. N. Y.,I1I.) 



Fig. 532. Megambonia suborbicularis, a, left side of internal mold showing muscular 

 impression ; b, profile of posterior side of same. (Pal. N. Y., III.) 



129. M. ovata Hall. Devonic. 



Distinguished by its nearly symmetrically ovate form with the 



