194 



and in having its ligamental pit always dilated in its 

 middle part. 



PectinidecB. — Ligament chiefly internal; the shell, in 

 general, regular and compact. 



1. Pedum. — An inequivalved, slightly eared bivalve, the 

 valves terminating at the base in a triangular disk, with a/ 

 furrow across that of the lower valve, and a cicatrix across 

 that of the upper, for the attachment of the hinge ligament; 

 the hinge toothless ; the lower valve notched in the left side, 

 near the hinge.* Recent. — PI. vii. fig. 7. 



2. Lima. — A longitudinal, subequivalved, eared bivalve, 

 gaping a little on one side of the valves ; the beaks sepa- 

 rated, their inner surface rather everted ; the hinge tooth- 

 less ; the hinge-pit, receiving the ligament, partly external. 

 Recent and fossil. — PI. vii. fig. 3. 



3. Plagiostoma. — A free, subequivalved, slightly eared 

 bivalve ; the base transverse and nearly straight ; the beaks 

 rather separated, their inner surfaces extended in flat, 

 transverse areas, disposed rather externally ; one straight, 

 the other inclined obliquely ; the hinge without a tooth ; a 

 conical hinge-pit under the beaks, partly internal, and 

 opening externally. Fossil. — PI. vii. fig. 12. 



4. Pecten. — A free, regular, unequal valved, eared bi- 

 valve ; the lower edge transverse and straight ; the beaks 

 contiguous ; the hinge without teeth, with a trigonal hinge- 

 pit entirely internal for the ligament. Recent and fossil. — 

 PI. vii. fig. 2. 



5. Plicatula. — An unequal valved bivalve, contracted 

 towards its base ; the upper margin rounded, and sub- 

 plicated ; the beaks unequal, and with no cardinal areas ; 



* The chief particulars respecting this genus are derived from 

 Mr. G. Sowerby's more extended, and, necessarily, more complete 

 description, in the work already referred to. 



