78 



PINNA.— ARCA.—PECTUNCULUS. 



The annual revenue from the pearl fishery of Ceylon is esti- 

 mated at about $80,000. 



18, The PiNN^ have two equal valves in form of a half-open 

 fan, gaping and united by a ligament along one of their sides. 

 A very large species is found in the Mediterranean, which lives 

 half-buried in the sand, and anchored by its byssus, the fila- 

 ments of which, being very strong, fine, and brilliant as silk, are 

 employed by the inhabitants of some parts of Calabria and Sicily 

 in the manufacture of precious stuffs. 



19. The Arcje are dis- 

 tinguished from all the 

 preceding by their equi- 

 valve shell. The hinge 

 which extends along the 

 side of the valves is 

 studded with a great many 

 little teeth, which fit into 

 the intervals of those of 

 Fig. 97. — ARCA. tj^g opposite side recipro" 



cally ; in some, the Arca properly so called {Jig. 97), the 

 hinge is rectilinear, and the shell is 

 longer in a direction parallel to 

 this junction ; in others, named 



hinge 



PECTUNCULrs (fig. 98), the 

 is curved, and the shell is lenticu 

 lar in form. The first are met with 

 near the shore in rocky situations, 

 and are ordinarily covered by a 

 hairy epidermis ; the latter live in 

 the mud. 



20. Finally, the Trigoni^, most 

 of the species of which are fossil, 

 are remarkable for their peculiar 

 hinge ; the right valve is furnished 

 with two projecting plates, crenulate 



Fig. 98. 



•PECTUNCULUS. 



18. What are the characters of the genus Pi'nna? (Pinna, Latin, x 

 fin.) 



19. What are the characters of the genus Arca ? What are the charac- 

 ters of the genus Pectu'nculus ? (Pectunculus, Latin, the diminutive of 

 pecteriy a comb.) 



20. What is Trigo'nia ? (From the Greek, trigonosy having three cor- 

 ners.) 



