SUBMYTILACEA. 



721 



Fig. 596. 



■Unio Valdensis. Wealden (Lower 

 Cretaceous). 



The shell in the genus Unio (fig. 596) is generally oval or 

 elongated, the beaks being placed towards the anterior end, and 

 a prominent external ligament 

 being present. The surface is 

 smooth, concentrically striated, 

 or in some cases ribbed, and 

 is covered with a thick horny 

 epidermis, which is often de- 

 stroyed over the beaks. The 

 hinge of the right valve has 

 two anterior lateral teeth, and 

 a long laminar posterior lateral 

 tooth; while that of the left 

 valve has a single anterior 

 lateral tooth, a cardinal tooth, 

 and two laminar posterior lat- 

 eral teeth. The most ancient fossil representatives of the River- 

 mussels appear in the Purbeck beds (Upper Jurassic), and other 

 forms (Unio Valdensis, &c.) are found in the Wealden deposits. A 

 considerable number of Tertiary types of Unio have been recognised, 

 principally in the Miocene beds, but the genus attains its maximum 

 at the present day. 



The genus Anodonta or Anodon comprises the living Swan- 

 mussels, and is distinguished from Unio by the tenuity of the shell 

 and the edentulous character of the hinge. The shell forms an 

 elongated oval, compressed in the young condition but convex 

 when old, with anteriorly placed beaks ; and it often attains a 

 considerable size. The earliest undoubted fossil forms of this 

 genus appear in the Eocene Tertiary ; but the Upper Devonian 

 rocks of Ireland and Scotland have yielded the remains of a large 

 Bivalve which has been referred here under the name of Anodonta 

 (Archanodontd) Jukesi. 



Family 4. Cardiniid^e. — This family was proposed by Zittel 

 to include a number of Bivalves, in which there is an equivalve, 

 oval, or trigonal shell, which is not nacreous internally, and in which 

 the surface is smooth or concentrically striated. The ligament is 

 external, the pallial line is entire, and the muscular impressions are 

 deep and simple. The cardinal teeth are generally small, but the 

 lateral hinge-teeth are more or less developed, and are often very 

 thick. The forms included in this family are all extinct, and present 

 relationships on the one hand to the Unionidce and on the other 

 hand to the Astartidcz. From the fossils with which they are 

 associated, they would appear to have been marine, or to have 

 inhabited brackish waters. 



In the genus Cardinia (fig. 597, b) the shell is trigonal or ovate, 

 vol. 1. 2 z 



