386 



MOLLUSCA 



SUB-KINGDOM VI 



byssus, smooth or radially sculptured, witli 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. 680). Shell thick, trigonal ; the front margin deeply 

 impressed. Upper Jura. 



Modiola, Lam. (Figs. 681, 682). Like Mytilus, but the beaks not terminal, an- 

 teriorly rounded and wider. Devonian to Recent. 

 Sub - genera : Modiolaria, Loven. Small, radially 



n i ' 



Pachymytilus petasus, d'Orb. 

 Rag; Coulange-sur-Yonne. %. 



FIG. 681. 



Modiola aspera, 

 Sow. Great 

 Oolite; Lan- 

 Coral- grune, Cal- 

 vados, i/i. 



FIG. 682. 

 Modiola inibricata, 



FIG. 683. 



Lithophagus inclusu?, Phill. 

 sp. Great Oolite ; Minchin- 



of the 



Sow. Middle Jura; Hampton, England. A, B, 

 Balin, near Cracow, Shell, i/j. C, Cast 



sculptured toward the ends, usually smooth toward the middle, Modioliform. Tertiary and 

 Recent. 



Crenella, Brown. Small, rounded, radially sculptured all over. Tertiary and Recent. 



Stavelia, Gray. Recent, Valves spirally twisted. 



Lithophagus, Meg. (Lithodomus, Guv.), (Fig. 683). Sub-cylindrical, with rounded 

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

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

 boniferous to Recent. 



Family 40. Dreissensiidae. Gray. 



Shell Mytiliform, equivalve, of prevailingly prismatic substance ; area linear, amphi- 

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

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

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

 as in Mytilus. Tertiary to Recent. 



Dreissensia, Van Ben. (Tichogonia, Rossm.), (Fig. 684). 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. 



Congeria, Partsch (Fig. 685). Sub - quadrate, heavy, large ; FIG. 684. 



myophores as in Miiti 



-n, * 



Europe. 



Dreissensiomya, Fuchs. Notable for being the only example of Mayence. Vi- 

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



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

 Tertiary and Recent. 



Very profuse in the Neocene of Eastern Dreissensia Bmnii, 



Faujas. Miocene ; 

 Weiss e nan, near 



