720 LAMELLIBRANCHIATA. 



tation, consisting sometimes of concentric lines, but more usually 

 taking the form of radial rows of tubercles or diverging ribs. By the 

 form of the shell and the ornamentation of the surface the species 

 of Trigonia may be divided into a number of minor groups. The 

 earliest types of Trigonia appear in the Lias, and the genus exhibits 

 an extensive development in the later Jurassic and Cretaceous for- 

 mations. A few Tertiary species are known, and the Australian 

 seas are inhabited by five existing forms. 



The Triassic genus Myophoria (fig. 594) comprises sub-triangular 

 shells, obliquely keeled, smooth, concentrically striated, or with a 

 partial development of radiating ribs. The left valve has three, and 

 the right two, cardinal teeth. In the genus Schizodus ( = the 

 Axinus of many authors, but not of Sowerby) are comprised a 

 number of Carboniferous and Permian Bivalves, in which the shell 

 (fig. 595) is obliquely ovate, and is in many respects externally 



Fig. 595.— Cast of Schiz- 

 odus obscurus, from the 

 Permian rocks, of the nat- 

 Fig.5g4.-My0/f1oriatineata. Trias. ural size. (After Zittel.) 



similar to Myophoria, except that the surface is smooth and non- 

 plicate, while the posterior side is bounded by an obscure oblique 

 ridge, and is not markedly angular. The right valve has two di- 

 verging teeth, and the left valve has a strong central tooth, with a 

 smaller marginal tooth on each side. The Devonian genus Cur- 

 tonotus and the Carboniferous Dolabra may be provisionally asso- 

 ciated with the present family. 



Family 3. Unionise. — In this family the foot is large and com- 

 pressed, not byssiferous except in the fry; the mantle-lobes are more 

 or less free, being usually united between the siphonal apertures ; 

 and there are two adductor muscles. The shell is ordinarily equi- 

 valve, nacreous internally, with a thick epidermis, and a large 

 external ligament. The hinge may be edentulous or furnished with 

 well-developed teeth, and the pallial line is entire. All the members 

 of this family are inhabitants of fresh water, and they are, therefore, 

 not known in the fossil condition except in fluviatile and lacustrine 

 deposits. If Anthracosia and Cai'bonicola be removed to the Car- 

 diniidce, the genera Unto and Anodonta are the only types of this 

 family which need consideration here. 



