256 Introduction 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 periostracnm, 

 over n layer of " cartilage" consisting of an iridescent material 

 soluble in caustic potass, which exhibits two sets of stria?, 

 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 

 (IVctunculus), 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 (\Ymis, &c.), rarely backwards (Lyriodon), or spirally 

 coiled (Isoeardia. Diccras, Chama). The right valve may be 

 smaller than the left (Os ; ndora, Lyonsia), or vice versa 



(t'hati and Corbula). The pcriostracum m; 



evam-M-enl or permanent, often bristled or tufted; sometimes 

 prolon 'inl the edge of the valves (Solemya, C 



daria), or extending over the siphon (Mya, Glycinu-ris, &c.) 

 The shell surface is marked with concentric lines of growth, 

 beginning around the umbo, also with radial lin< 



, or s] tines, corresponding to irregularities in the mantle 

 border. The valves may be capable of perfect close 

 may -ape posteriorly for the emission of the siphons, ; 

 in front (Roccllaria, 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, Crcnatnla. 



f In Donnx, Glycimevis, Solemya, &c., the umbo is nearer the hinder 

 than the front edge of the shell. 



J Somclirncs the teeth are formulated e. g. C -^ / - ' ^. -' ' 



ri-ht valve 2 , anterior on the right valve I 



(meaning Cardinals, - , laterals - , 



left valve 3 autenor on ihe kti \ 



posterior right I 

 posterior left 2 



