256 Introdtiction to Animal Morphology. 



hinge, outside which is an elastic ligament, which 

 during life keeps the valves open. 



This consists of an outer thickening of the periostracum, 

 over a layer of "cartilage" consisting ofan iridescent material 

 soluble in caustic potass, which exhibits two sets of striae, 

 vertical and horizontal ; this lies in a pit or chondrophore, 

 often elongated and spoon-like (Mya, Mactra), sometimes 

 multiple."^ Each shell valve has on its upper edge, over- 

 hanging the hinge, a beak or umbo, which may be central 

 (Pectunculus), but is usually nearer the anterior (mouth) side, 

 which is thus shorter than the posterior, which bears the 

 ligament.f The umbo may be straight (Mytilus), or bent 

 forwards (Venus, &c.), rarely backwards (Lyriodon), or spirally 

 coiled (Isocardia, Diceras, Chama). The right valve may be 

 smaller than the left (Ostrea, Pandora, Lyonsia), or vice versa 

 (Chamostrea and Corbula). The periostracum may be 

 evanescent or permanent, often bristled or tufted, sometimes 

 prolonged beyond the edge of the valves (Solemya, Cyrto- 

 daria), or extending over the siphon (Mj-a, Glycimeris, &c.) 

 The shell surface is marked with concentric lines of growth, 

 beginning around the umbo, also with radial lines, ridges, 

 scales, or spines, corresponding to irregularities in the mantle 

 border. The valves may be capable of perfect closure, or 

 may gape posteriorly for the emission of the siphons, rarely 

 in front (Rocellaria, Gastrochaena) for the protrusion of the 

 foot, or ventrally (Galeomma). At the hinge there are teeth 

 in each valve fitting into corresponding hollows in the neigh- 

 bouring valve. Those directly under the beak are cardinal ; 

 those in front or behind it are lateral.\ The cardinals may 



* Iridina, Crenatula. 



t In Donax, Glycimeris, Solemya, &c., the umbo is nearer the hinder 



than the front edge of the shell. 



r. 2 r.fl.i r.p.x 

 % Sometmies the teeth are formulated e. g. C -— ^ / . - — 



right valve 2 anterior on the right valve i 



(meaning Cardmals, — ^- ; , laterals — -. — -r— , 



^ left valve 3 antenor on the leit valve 2 



posterior right i 

 posterior left 2 



