90 PAL/EONTOLOGY 



forms of Inoceramus of the Upper Cretaceous, or Hippopo- 

 dium of the Lias. 



Ostrea is possibly as old as Carboniferous, and is 

 abundant from the Jurassic onwards. Gryphaa is mainly 

 Jurassic; Exogyra, Upper Jurassic and Cretaceous. 



5. Mytilacea. Equivalve; heteromyarian ; interior 

 nacreous ; umbo anterior ; hinge-line short, without 

 teeth, or with very obscure cardinal teeth ; ligament 

 opisthodetic ; no ears or byssal notch, but a slight byssal 

 gape ; shell elongated in an oblique direction. Ornament 

 concentric or partly radial. Marine or fluviatile. 



This sub-order is typified by the common marine 

 mussel, Mytilus. The shell is always elongated in an 

 oblique direction and tends to a triangular shape, the 

 apex being formed by the umbo, which is at or near the 

 anterior end of the hinge-line and is never conspicuous, 

 the base of the triangle being the posterior border. The 

 area is generally obscure, but apparently amphidetic, 

 but the ligament is opisthodetic. Hinge-teeth are want- 

 ing or indistinct. Early genera such as Modiolopsis 

 (Ord.-Sil.) and Myoconcha (Carb.-Cret.) are less hetero- 

 myarian than the rest. Modiola (Fig. 25, /, Dev.-Rec., 

 maximum in Jurassic) is more quadrilateral than Mytilus, 

 the beaks not being so far forward. Lithophagus [Litho- 

 domus] (Carb.-Rec.) is flask-shaped, and bores into corals, 

 thick shells such as Perna, or rocks on the sea-bottom : 

 casts of its borings sometimes puzzle the fossil-collector. 

 Dreissensia (Eoc. Rec.) and Congeria (Mio.-Plio.) are fresh- 

 water forms, having the anterior adductor borne on a 

 myophoric plate. It has been suggested that these are 

 not true Mytilacea, but homceomorphs derived from some 

 other stock, but there is little evidence for this. 



