ASIPHONIDA. 



491 



Fig. B55.Myop7wria lineata. Trias. 



respects closely allied to the preceding, but having the pos- 

 terior side marked by an obscure oblique ridge, and not so 

 markedly angular as in Myophoria. 



We may also provisionally associate with the above the 

 Palaeozoic genera Curtonotus of the Devonian, Anodontopsis of 

 the Silurian, and Dolabra 

 of the Carboniferous, the 

 structural characters of 

 which are still but very 

 imperfectly known. 



FAM. 6. UNIONISE. 

 Shell usually equivalve, 

 with a large external liga- 

 ment. Anterior hinge- 

 teeth thick and striated ; 



posterior laminar or wanting. Mantle-lobes united between 

 the siphonal apertures. Foot very large, compressed, bys- 

 siferous in the fry. All the members of the Unionidce are 

 inhabitants of fresh water, and they are therefore not known 

 as fossils except in fluviatile and lacustrine deposits. The 

 principal fossil genera of the family are Unio, Anodon, An- 

 thracosia, and Carbonicola. 



In the genus Unio (fig. 356) the shell is oval or elongated, 

 somewhat resembling that of a mussel (hence the name of 

 Eiver - mussels commonly 

 given to the Uhios). The 

 species of this genus ap- 

 pear to commence in the 

 Lower Cretaceous rocks, 

 and they are very abun- 

 dant at the present day. 

 The beaks of fossil speci- 

 mens are often deeply 

 eroded, as are those of 

 living forms. 



The genus Anodon or 

 Anodonta (Swan -mussels) 



closely resembles Unio, but the shell is edentulous. The 

 earliest undoubted fossil forms occur in the Lower Tertiaries 



Fig 356. Unio Valdensis. Wealden (Lower 

 Cretaceous). 



