CLASS T 



PELECYPODA 



463 



Mytilus Liim. (Fig. 728). Shell elongated, thiii, witli terminal pointed beaks ; 

 valves wider and rounded beliind, gaping a little for tlie byssus, smooth or radially 

 sculptured, witli sniootli niargins, conspiciious epiderinis, and a thin nacreous layer ; 

 hinge with a few small teeth iinder tlie beaks, or edentulous. Trias to Recent. 



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



impressed. Upper Jura. 



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



Pacliy mytilus petasus d'Orb. Coral-Rag ; 

 Coulaiige-.sur-Yonne. 2/3. 



Fi(!. 730. 



Modiolus OS- ^^'^- "^l- 



per US Sow. Modiolus imhri- 



Great Oolite ; cahis Sow. Middle 



Langrune, Cal- Jura ; Baiin, near 



vados. i/x- CracoWjAn.stria.i/i- 



Fio. 732. 



Lithopliagus inclusus (Ph\l\.). 

 Qreat Oolite ; Minchinhamp- 

 ton, England. A, B, Shell, 

 1/1. ü, Mould of the burrow. 



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



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

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

 rounded, radially sculptured all over.' Tertiary and Recent. 



Stavelia Gray. Recent. Valves spirally twisted. 



Litliophagus Meg. {Lithodomus Cuv.) (Fig. 732). Sub-cylindrical, with rounded 

 ends ; perforating coral, limestone and other substances, in wliieli 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 prismatic suhstance ; area linear , amphi- 

 detic ; ligament sub-internal ; anterior adductor and pedal protractors inserted on a 

 myophoric septum ; mantle lohes united to form anal and hranchial siphons, and also 

 ventrally with 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, with 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 Mytilopsis. Very profuse in the Neocene of Eastern 

 Europe (Congeria beds). 



IJreissensiomya Fuchs. Notalgie for being the only example of the 

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



Septifer Recluz. Valves wuth strong radial or divaricate sculpture. 

 Tertiary and Recent. 



Fig. 733. 



Dreissensia brardi 

 Faujas. Miocene ; 

 Weissenau, near 

 Mayence. Vi- 



Marine 



