CLASS I 



PELECYPODA 



463 



Mytilus Linn. (Fig. 728). Shell elongated, tliin, with terminal pointed beaks ; 

 valves wider and rounded behind, gaping a little for the byssus, smooth or radially 

 sculptured, with smooth margins, conspicuous epidermis, and a thin nacreous layer ;' 

 hinge with a few small teeth under the beaks, or edentulous. Trias to Recent. 



Pachymytilus Zitt. (Fig. 729). Shell thick, trigonal; the front margin deeply 



imj^ressed. Upj)er Jura. 



Modiolus Lam. (Figs. 730, 731). Like Mytilus, ■ 



Pachymytilus petasus d'Orb. Coral-Rag ; 

 Coulange-sur-Yonne. 8/3. 



Modiolus as- 

 Ijerus Sow. 

 Great OolitR ; 

 Langrune, Cal- 

 vados. 1/1. 



Modiolns imbrl- 

 cntus Sow. Middle 

 Jura ; Balin, near 

 Cracow, Austria.i/i. 



Lithopliagus inclusus (Phill. ). 

 Great Oolite ; Miiichinhamp- 

 ton, England. A, D, Shell, 

 l/l- t', Mould of the buiTOw. 



but the beaks not terminal, edentate, anteriorly rounded and wider. Devonian to Eecent. 



Subgenera : Modiolaria Loven. Small, radially sculptured toward the ends, usually 

 smooth toward the middle, modioliform. Tertiary and Recent. Crcnella Brown. Small, 

 rounded, radially sculptured all over. Tertiary and Recent. 



Stavelia Gray. Recent. Valves spirally twisted. 



Litlioplmgus Meg. {Lithodomus Cuv.) (Fig. 732). Sul) -cylindrical, with rounded 

 ends ; perforating coral, limestone and other substances, in which the animal forms 

 flask-shaped excavations ; moulds of the latter are often found in the fossil state. 

 Carboniferous to Recent. 



Family 40. Dreissensiidae Gray. 



Shell mytiliform, equivalve, of prevailingly jJrismatic substance ; area linear, amrphi- 

 detic ; ligament suh-infernal ; anterior adductor and pedal protractors inserted on a 

 myophoric septum ; mantle lobes united to form anal and branchial sijjhons, and also 

 ventrally ivith a pedal opening ; pallial line usually simple; gills reticulate ; otherwise 

 as in Mytilus. Tertiary to Recent. The relations of this family are in question. 



Dreissensia Van Ben. (Tichogonia Rossm.) (Fig. 733). Smooth, without a pearly 

 layer, w^ith a single apical septum ; fluviatile and estuarine. Eocene 

 to Recent ; Europe. 



Mytilopsis Conr. Mytiliform, small, thin ; myophore for the 

 pedal protractor distinct from that which supports the anterior 

 adductor. Tertiary and Recent ; America. 



Gongeria Partsch (Fig. 734). Sub-quadrate, heavy, large ; 

 myophores as in Mrjtilopsis. Very profuse in the Neocene of Eastern 

 Europe (Gongeria lieds). 



Dreissensiomya Fuchs. Notable for being the only example of the Mayeiice'"' Vi 

 Mytilacea with a distinct pallial sinus. Miocene ; Eastern Europe. 



Septifer Recluz. Valves -with strong radial or divaricate sculpture. Marine 

 Tertiary and Recent. 



Pig. 733. 



Drcisseiisia brardi 

 Faujas. Miocene ; 

 Weissenau, near 



