LAMELLIBRANCHIATA. 27 



ranean (Temple of Serapis at Pozzuoli). Dreyssena polymorplia Fall., has 

 gradually extended over many freshwater systems of Germany. 



Fam. Arcacese (Archemuscheln). Shell thick, equivalved with well- 

 developed hinge, and covered by hairy epidermis. The two adductors form 

 two equally large anterior and posterior muscle impressions. Area Nofs L., 

 Mediterranean. Pectunculus pilows L., Mediterranean. 



The Trigoniadae (Trigoniacea) are allied here. Trigonia pectlnata Lam. 



Fam. TJnionidae (Najades), Freshwater Mussels. With long equivalved but 

 not equisided shells, which are covered externally by a strong smooth usually 

 brown epidermis, and internally by a mother-of-pearl layer. One of the 

 muscle impressions is divided. Foot with cutting edge ; gills fused behind the 

 foot. The outer gill plates also function as brood-pouches for the developing 

 eggs. They live in standing or running water. 



Anodonta cygnea Lam., in ponds. A. anatina L., more in rivers and brooks. 

 Unio pictorum L., (Malermuschel). Unio tumidus Retz.^ batavus Lam. Mar- 

 garitana margaritifera Retz. (Flussperlmuschel), in mountain streams of 

 South Germany, especially in Bavaria, Saxony, and Bohemia. 



II. SIPHONIATA, 



Part of the mantle edges fused, with elongated tubular siphons. 



Fam. Chamidae (Chamacea) (Gienmuscheln). Shell unequivalve, with 

 strongly developed cardinal teeth and simple pallial line. The mantle edge fused, 

 except at three points, viz., the opening for the foot, the dorsal (cloaca!) and 

 ventral (inhalent) siphons. Chama Lazarus Lam. 



The Tridacnidae are closely related to the above. Tridacna gigas L. Hip- 

 popits maciilatus Lam. Indian Ocean. 



Fam. Cardiidae (Cardiacea) Cockles. Shell equi valve, fairly thick, heart- 

 shaped and arched, with large incurved umbones, external ligament, and strong 

 hinge formed of several teeth. Siphons short. Foot powerful and bent elbow- 

 like, serves for swimming ; passes out through anterior slit. Cardium cdule L., 

 North Sea and Mediterranean. Hemicardium cardissa L., East Indies. 



Fam. Lucinidae (Lucinacea). Shell circular, free, closed, with one or two 

 cardinal teeth, and a second quite rudimentary lateral tooth. Pallial line 

 simple. Mantle open in front, prolonged behind into one or two siphons. 

 Lucina lactea Lam. Mediterranean. 



Fam. Cycladidae.* Shell equivalve, free, swollen, with external ligament and 

 thick horny epidermis. Mantle with two (rarely one) more or less fused 

 siphons. Live in fresh water. Cy das cornea L., Pisidium Pf. Corbicula Miihlf. 



Fam. Cyprinidae. Shell regular, equivalve, elongated to an oval, closed, with 

 thick and strong epidermis. One to three principal cardinal teeth, and usually a 

 hinder lateral tooth. Pallial line simple. Mantle edges fused to form two 

 s iphonal openings. Cyprina islandica Lam., Isocardia cor L. Mediterranean. 



Fam. Veneridae. Shell regularly round, or oblong with three diverging car- 

 dinal teeth on each valve. Pallial line bent in. Siphons of unequal size, fused 

 at the base. Venus verrucosa L., Mediterranean. Cytherca CMone L., edible, 

 Mediterranean. C. Dione L., Atlantic Ocean. 



Fam. Mactridae (Fig. 499). Shell trigonal, equivalve, closed or slightly 



* Fr. Leydig, "Anatomie und Entwickelung vonCyclas." Mailer's ArcMv, 

 1835. 



