MOLLUSCA—PELECYPODA. 367 



absent or very obscure. Surface usually crossed by strong, plate-like 



concentric lamellae 99 



99. Beaks small, not curved LXXIX. Ostrea 



99. Beaks prominent, strongly incurved .jj 



jj. Beaks curved in the median plane LXXX. Gryphcea 



jj. Beaks curved to right or left of median plane.XXXXI. Exogyra 



f. Shell not ostreiform 000 



000. Wings or ears two, a small anterior and a large posterior (when 



small usually called ear ; when large, wing) kk 



kk. Surface radially sculptured 7f 



7f. Hinge area longitudinally grooved on both sides of the beak. 6* 



6*. Adductor muscle scars two j ff 



j // . Pallial line and umbonal muscles indicated by man] 



small, deep pits XLVI. Limoptera 



3". Shell without pits XLV. Pterinea 



6*. Adductor muscle scar one 4" 



4 n '. Hinge line forming greatest width of shell. 



XCIV. Pterinopecten 

 4". Hinge line not equalling greatest width of shell. 



XCV. Ly Hope den 

 7f. Hinge area not grooved longitudinally ; also muscle scars 



practically but one (see LXVIII. Pteria) 7* 



7*. Umbonal ridge very strong LXIII. Conocardium 



7*. Umbonal ridge weak or wanting 5" 



j // . Anterior extremity of ear pointed. 



LXXI. Ptychopteria 

 j // . Anterior extremity of ear rounded. 



LXX. Actinopteria 

 kk. Surface without radial sculpture ( where muscle scars are reduced 



practically to one, see LXVIII. Pteria') 8f 



8L Shell strongly oblique 8* 



8*. Anterior ear acute 6" 



6 // . Hinge longer than body of shell.. .LXIX. Pteronites 

 6 f/ . Hinge shorter than body of shell.. .XLIX. Leptodesma 



8*. Anterior ear rounded.. .„ XLVII1. Leiopteria 



8f- Shell but slightly oblique ; wing elongate, narrow. 



XLVII. Actinodesma 

 000. Ears two, the anterior much the longer ; byssal notch edged with a 



comb-like series of small teeth XCV1I1. Chlamys 



000. Ears two, the anterior longer ; byssal notch smooth. 



XCIX. Camptonectes 



000. Ears subequal //. 



//. Ears very distinct 9f 



9f. Shell with small internal ribs radiating from beneath the beaks 



CI I. Amusium 



9f. Shell without internal ribs 9* 



9*. Ears diverging at a sharp angle above the beaks. 



C. Entolium 



9*. Ears not diverging at a sharp angle 7 // 



7 // . Ligament in shallow grooves roughly paralleling the 

 hinge margin d // . 



