ORDERS AND FAMILIES. 87 



MYIDJE. Shell inequi valve, oblong, and gaping widely on the posterior 

 side, and often, but more narrowly, on the anterior side also ; pallial 

 line broad and deeply sinuated, muscular scars large and distinct ; 

 ligament on a prominent shelf ; periostracum wrinkled and extensive. 

 My a, 143 to 145 ; Corbula, 146 ; Lutmria, 147, 148. 



PANDORID^:. Shell inequivalve, semilunar, pearly, and gaping at posterior 

 end, which is flexuous and elongated ; ligament entirely internal, long 

 and oblique and often with calcareous ossicle ; pallial line entire or 

 with slight sinus. Pandora, 172. 



PETRICOLID^:. Shell oval or elongated, thin, gaping slightly behind ; 

 hinge with three teeth in each valve, the external often obsolete ; 

 pallial sinus deep and well marked. Petricola, 128. 



PHOLADHXE. Shell gaping at both ends without hinge or ligament ; one 

 or more accessory dorsal pieces ; an internal apophysis from the beak 

 cavity ; dorsal margin partly reflected over the beaks. Pholas, 160 to 

 163 ; Pholadidea, 164 ; Xylophaga, 165. 



PSAMMOBIID^. Shell nearly equilateral, long oval, compressed, striated 

 concentrically and longitudinally, posterior side somewhat truncate 

 and gaping ; ligament external and prominent ; two cardinal teeth in 

 each valve, the posterior one in the left valve being small and set 

 obliquely, the others being cloven ; pallial sinus deep. Psammobia, 

 139 to 142. 



SCROBICULARIID.E. Shell equivalve, white, gaping at posterior end ; beaks 

 turned towards posterior side, and almost contiguous ; hinge teeth 

 weak, and consisting of two small cardinals in right valve and one in 

 the left valve ; ligament in an internal cavity. Scrobicularia, 99 to 103. 



SOLENID^;. Shell equivalve, narrow, long, and gaping at both ends ; 

 hinge strengthened inside by a ridge ; teeth usually two and three, the 

 cardinals being shaped like thorns ; ligament external; beaks flattened. 

 Sohcurtus, 149 and 150; Ceratisolen, 151 ; Solen, 152 to 155. 



TELLINID^. Shell equivalve, compressed ; ligament external, and on 

 shortest side of shell ; beaks incurved and nearly straight ; siphons 

 separated, slender, and very long ; mantle widely open in front, and 

 with fringed margin ; foot and labial palpi large ; adductor scars far 

 apart; pallial impression deeply sinuated. Tellina, 90 to 97; 

 Gastrana, 98. 



TEREDINID^. Shell small, continued into a long, calcareous tube ; valves 

 deeply notched ; an internal apophysis; siphons long, and ending with 

 two calcareous pallets. Teredo, 166 to 171. 



. Shell equivalve, oblong, inequilateral, pearly within; large 

 external ligament ; anterior hinge teeth thick and striated, posterior 

 teeth often wanting ; all teeth occasionally rudimentary. Unio, 55 to 

 57, and Anodonta, 58, 59. 



Shell regular and solid ; hinge usually with three large, 

 diverging cardinal teeth, laterals variable ; ligament external ; adductor 

 scars oval and distinct, pallial line deeply sinuated. Circe, 113 ; Vemts, 

 114 to 121 ; Lucinopsis, 122 ; Tapes l 123 to 126; Venernpis t 127. 



