PERKA— ETHERIA.—AVICULA. 



■rs 



will 



Fig. 94. PERNA, 



mention Etheria, Avicula, 



13. Pern A (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- 

 sertion of these organs, we 

 Pinna, Area, and Trigonia, 



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

 large, 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 Avici] LiE are recognised by their inequivalve shell, 

 with a rectilinear hinge furnished with a narrow ligament, and 



frequently elongated 

 into wings at its two 

 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. ^^^g^g jg ^ ^Qt^h 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 

 shell-fish.) 



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

 Greek, nithb, I shine.) 



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

 bird ; 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 

 from Avicula? (Meleagri'na; from the Greek, meleag'ris, a guinea-hen.) 



