PERNA ETHERIA. AVICULA 75 



13. PERNA (fg. 94), in 

 which the hinge is composed 

 of several ligaments inserted 

 into particular pits ; besides 

 a great many other shells, 

 either recent or fossil. 



Among the Ostracea 

 provided with two distinct 

 muscles, and the shells con- 

 sequently marked, on the 

 internal surface of each 

 valve, by two irregular sur- 

 faces (muscular depressions) 

 corresponding with the in- Fi>. 94. PERNA. 



sertion of these organs, we will mention Etheria, Avicula, 

 Pinna, Area, and Trigonia. 



14. The EtheritE are very analogous to oysters : their shell is 

 , inequivalve, very irregular, without teeth at the hinge and 



provided with a ligament, which is partly internal and partly- 

 external. They are very rare shells, and have been overlooked 

 by travellers from being attached to rocks at considerable depths.. 

 They inhabit the East Indian seas. 



15. The AVICUL.E are recognised by their inequivalve shell, 

 with a rectilinear hinge furnished with a narrow ligament, and 



frequently elongated 

 into wings at its iwo 

 extremities (fg- 95). 

 The body of these 

 mollusks is very small 

 and prolonged into a 

 vermiform and coni- 

 cal foot, and furnished 

 with a byssus, for 



the passage of which 

 Fig. 95. AVICULA. there js a notch in the 



side of the shell. The anterior adductor muscle is extremely small 

 and the labial appendages very large. These mollusks have 



13. What is the genus Per'na? (Perna, Latin, a gammon of bacon, a 



14. What are the characters of the genus Ethe'ria ? (Etheria, from the 

 Greek, aithb, 1 shine.) 



15. How is the Avi'cula recognised ? (Avicula, from the Latin, avis, a 

 trd ; because, when the shell is widely open, it bears a remote resemblance 

 to a bird with spread wings) How does the genus Meleagri'na differ 

 trom Avicula? (Meleagri'na; from the Greek, meleng'ris, a guinca-hen.) 



