226 MOLLUSCA. LAMELLIBRANCHIA 



the umbo to the posterior angle. Surface generally with concentric 

 grooves. Umbones prominent. Right valve larger and more convex 

 than the left, and with a strong cardinal tooth in front of the 

 ligament-pit, and also a posterior cardinal tooth ; left valve with 

 a spoon-like process for the internal ligament, and one posterior 

 cardinal tooth. Adductor impressions well marked. Pallial line 

 slightly sinuous posteriorly. Trias to present day. Ex. C. Jicus, 

 Barton Beds ; C. sulcata, Becent. 



Panopea. Shell equivalve, inequilateral, oblong, thick, con- 

 centrically striated, gaping at each end, especially at the posterior. 

 Ligament external, on a prominent ridge. One cardinal tooth in 

 each valve. Pallial sinus very deep. Cretaceous to present day. 

 Ex. P. faujasi, Coralline Crag to present day. 



Saxicava. Shell small, more or less oblong, gaping ; umbones 

 anterior. Ligament external. Teeth absent in the adult, one or 

 two cardinals present in the young. Pallial line not continuous, 

 sinuous. Saxicava bores into rocks, etc. Jurassic to present day. 

 Ex. S. rugosa, Coralline Crag to present day. 



Pholas. Shell elongate, cylindrical, gaping at both ends. 

 Surface with spiny ridges, best marked in front. On the dorsal 

 region are one or more calcareous plates. No teeth ; no ligament. 

 In the interior, under the umbones, is a process for the insertion of 

 the muscle of the foot. Pallial sinus very deep. Pholas bores into 

 rocks, etc. Lias to present day. Ex. P. cylindrica, Bed Crag ; 

 P. dactylics, Becent. 



Teredo. Shell more or less globular, gaping at the ends, 

 valves tri-lobed, with concentric stria?. In the interior, under the 

 umbones, is a long narrow plate for the insertion of the pedal muscle. 

 Posterior part covered by a long, calcareous tube, which is sub- 

 cylindrical, straight or curved, and often with partitions. Teredo 

 perforates wood. Jurassic to present day. Ex. T. norvegica, Coral- 

 line Crag to present day. 



6. Hinge desmodont. Ligament usually behind the 

 umbones. Two nearly equal adductors. Pallial line 

 usually with a sinus, but sometimes sinuous only. Valves 

 generally somewhat unequal. 



