MOLLUSCA. LAMELLIBRANCHIA 217 



Alectryonia. Similar to Ostrea. Both valves with coarse 

 angular folds ; edges of valves toothed. Trias to present day. Ex. 

 A. gregaria, Corallian ; A. frons, Lower Chalk. 



Grryphaea. Shell similar to Ostrea, but free in the adult 

 stage ; left valve large and convex, with a prominent incurved umbo. 

 Right valve flattened or concave. Lias to present day. Ex. G. 

 arcuata { = incurva\ Lias. 



Exogyra. Similar to Ostrea. Shell fixed by the left (larger) 

 valve. Right valve flat, resembling an operculum. Umboues more 

 or less spiral, directed posteriorly. Upper Jurassic to Chalk. Ex. 

 E. cohcmba, Upper Greensand. 



4. Hinge schizodont, or formed of thick, irregular 

 teeth. Valves equal. Two nearly equal adductors. 

 Ligament usually external and behind the umbones. 

 Pallial line simple. Inner layer of shell usually pearly. 



Trigonia (fig. 93 B, C). Shell thick, ornamented with rows 

 of tubercles or with concentric (sometimes radiating) ribs ; trigonal, 

 very inequilateral, anterior margin rounded, posterior produced and 

 angular. Generally with a ridge extending from the umbones to the 

 posterior border, cutting off a portion which has a different orna- 

 mentation. Umbones anterior, directed posteriorly. Cardinal teeth 

 diverging, grooved, two in the right valve, three in the left, the 

 central tooth in the latter being bifid. Ligament marginal, thick. 

 Adductor impressions deep, the anterior smaller than the poste- 

 rior, and placed near the umbones. A pedal impression in front of 

 the posterior adductor of each valve and also one in the umbo of 

 the left valve. Pallial line simple. Interior of shell nacreous. Lias 

 to present day. Ex. T. costata, Inferior Oolite to Cornbrash ; T. cla- 

 vellata, Corallian. 



Schizodus. Similar in form to Trigonia ; shell thin and 

 smooth, umbones placed anteriorly. Three teeth in each valve, the 

 middle one being a cardinal ; the anterior lateral inconspicuous in 

 the right valve. Adductor impressions shallow. Carboniferous and 

 Permian. Ex. S. obscurus, Permian. 



Myophoria. Allied to Schizodus. Shell oval, triangular, or 

 trapezoidal. Umbones anterior, often with a ridge extending to the 



