386 MOLLUSCA SUB-KINGDOM VI 
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. 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: JJodiolaria, Lovén. Small, radially 

Fic. 681. Fic. 683. 
Fic. 682. 

Modiola aspera, Lithophagus inclusus, Phill. 
Fic. 680 Sow. Great Modiolaimbricata, sp. Great Oolite; Minchin- 
a aad Oolite; Lan- Sow. MiddleJura; hampton, England. 4A, B, 
Pachymytilus petasus, @Orb. Coral- grune, Cal- Balin,nearCracow, Shell, 1/j. C, Cast of the 
/ 
Rag; Coulange-sur-Yonne. 2/3. vados. 1/,. Austria. 1/}. burrow. 
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, Cuv.), (Fig. 683). Sub-eylindrical, 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 ; Fic. 684. 
myophores as in Mytilopsis. Very profuse in the Neocene of Eastern _Dreissensia Brandt, 
= y “aujas. Miocene ; 
Europe. Weissenau, near 
Dreissensiomya, Fuchs. Notable for being the only example of Mayence. "1. 
the Mytilacea with a distinct pallial sus. Miocene ; Eastern Europe. 
Septifer, Recluz. Valves with strong radial or divaricate sculpture. Marine. 
Tertiary and Recent. 
