THE BIVALVES 



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family, in which the shells are very flat, without any byssal opening; the valves 

 being thin, somewhat nacreous, with two long divergent hinge teeth to which 

 the ligament is attached. About half a dozen species from the Indo-Pacific 

 Ocean are known. P. sella has a somewhat wavy or cockled appearance, and 

 is known as the saddle oyster, on account of its saddle-like form. The arks 

 {Arcidce) are nearly all strong heavy shells, generally equivalve, but in some 

 instances more or less inequivalve; and always remarkable for their straight 

 hinge line, furnished with very numerous teeth. The form is variable; but the 

 valves are generally radiately ribbed, and more or less covered with a periostracum, 

 which may be smooth and thin, or thick, and very rugged. They may either meet 

 all round when closed, or may gape ventrally for the passage of a byssus. There are 

 two adductors far apart, and the pallial line is simple. The species both recent 

 and fossil are very numerous; and at the present time occur in all seas, some having 

 a very wide distribution. For in- 

 stance, the little Area lactea, 

 which is found on the British 

 coast, also occurs in the Philip- 

 pine islands, the Red Sea, South 

 Africa, Ascension island, and the 

 Mediterranean; and another 

 species (A. corpulenta) has been 

 dredged off North Australia, 

 south of Amboyna, in mid-Pacific, 

 and off the coast of Chili, in 

 depths ranging from two hundred 

 to two thousand four hundred 

 and twenty-five fathoms. In the 

 allied Pectunculus the shell is 

 rounded, strong, equivalve, with 

 the hinge teeth in a curved line; 

 the outer surface being sometimes 

 covered with a velvety or pilose 

 periostracum. Limopsis some- 

 what resembles Pectunculus in 

 form, but the shells are more 

 compressed and clothed with a 

 fibrous periostracum, and the 

 animal spins a byssus. Several of 

 the species have been dredged at 

 enormous depths in the Atlantic. 

 The genus Trigonia, represented 

 by about half a dozen species 

 occurring on the shores of 

 Australia, is all that now remains of the large family Trigoniidce, of which several 

 other genera, with a very large number of species, occur fossil in the Secondary 



J v 



COMMON MUSSEL (Mytilus edulis) , CLOSED AND ATTACHED 

 BY THE BYSSUS (natural size). 



