BIVALVES. 



405 



some instances more or less inequivalve ; arid 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 Ambojma, 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 com- 

 pressed 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 Trigoniidcc, of which several 

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

 and Tertiary rocks. The valves of Trigonia are beautifully pearly within, equal, 

 radiately ribbed, with an external ligament, and a few strong striated divergent 

 hinge-teeth. The umbones are inclined posteriorly a very unusual feature in 

 bivalves. The foot of the animal is large and powerful, used in crawling and 

 leaping, and without a byssus. In some of the Jurassic rocks of Weymouth 

 trigonias form a bed several feet in thickness. Mussels (family Mytilidce) are 

 such well-known shells that a description is unnecessary. They are found all over 



7 



COMMON MUSSEL (Mytilus edulis\ CLOSED AND ATTACHED BY THE 

 BYSSUS (nat. size). 



