LAMELLIBTIANCHIATA. 515 



rest on the right valve : the A?iomicB are attached by degenerated 

 muscular fibres passing through a hole or notch in that valve. All 

 these shells are called iuequivalve. The bivalve is called close when 

 the valves fit accurately ; it is gaping if part of the borders do not 

 come into contact when the shell is shut: in Gastroclicena this per- 

 manent opening is anterior, and serves for the passage of the foot ; 

 in Mya it is posterior, and serves for the passage of the siphons ; in 

 Byssoarca it is at the ventral border, and serves for the passage of 

 the byssus ; in Soleii and Glycymeris the shell gapes at both ends. 



The outer surface of the shell is usually coated by an uncalcified 

 layer of albumen called the periostracum or epiderm, continuous with 

 the mantle. This surface is variously ornamented in many species ; 

 sometimes with ridges or "ribs," which may be either concentric and 

 conformable with the layers of growth, or radiating from the um- 

 bones to the free margins of the valves ; and the ribs may be 

 direct, bent, oblique, or wavy. In Tellina fabula the sculpturing 

 is confined to the right valve. In many species of Pholas, Teredo, 

 and Cardiumy the surface is divided into two areas by a dorso- 

 ventral furrow, or by a change in the direction of the ribs. The 

 thorny oysters {Spondylus), are so called on account of the spines 

 which project from the rib-lines ; they are longest and in greatest 

 number upon the non-adhering valve. In some conditions of the 

 shell the spines expand into foliaceous forms. 



The part where the two valves are joined together is called the 

 hinge. The cardinal or hinge-line is short in Vulsella, long and 

 straight in Avicula and Area, of moderate length and curved in 

 most genera. The locomotive bivalves have, generally, the strongest 

 hinges : however, a very well-developed example of the hinge 

 mechanism may be studied in the Spondylus. Certain projections 

 or teeth of one valve interlock into cavities in the opposite valve. 

 The central teeth, usually beneath the umbo, are called cardinal 

 teeth ; those on each side are lateral teeth. In Alasmodon and Kellia 

 only lateral teeth, in many bivalves only cardinal teeth, are present : 

 the teeth are apt to become thickened and even obliterated by age, 

 through irregular growth or by the encroachment of the hinge-line. 

 Many of the fixed and boring shells are edentulous. 



The soft mechanism of the hinge consists of the ligament and 

 spring. The ligament is a tough thickened portion of albuminous 

 matter, like that of the periostracum, and is usually attached to 

 ridges on the outer (dorsal) part of the hinge-margin, behind the 

 umbones ; it is consequently stretched by the closing of the valves. 

 The spring, sometimes called " internal ligament," and (though im- 



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