LAMELLIBRANCI1IATA. 27 



ranean (Temple of Serapis at Pozzuoli). Dr, //A.W //it />1 i/mnrphti Fall., has 

 gradually extended over many fi-esh water systems of Germany. 



Fam. Arcaceae (Archenmscheln). Shell thick, equivalved with well- 

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

 two equally large anterior and posterior muscle impressions. . I rcn .\oic L., 

 Mediterranean. Pi'ctuitciiln* j/ihisnx L., Mediterranean. 



The Trigoniadae (Trigoniacea) are allied here. Trigonia jin-t i////f// Lam. 



Fam. Unionidae (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 cygiiea Lam., in ponds. A. laidtina L., more in rivers and brooks. 

 1'n in ji'ictfiniiii L., (Malermuschel). Unio tumid us Retz., batarus Lam. J/ar- 

 yaritdtid, ^nargaritifera 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) (Gieumuschelu). Shell uuequivalve, with 

 ^trongly 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. C'Jiama Lazurus Lam. 



The Tridacnidae are closely related to the above. Ti'idacnu ffi'/as L. Hip- 

 pi\pux mdculdtux 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. Curdinw cdule L., 

 North Sea and Mediterranean. Hemicardium cardixsa 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. 

 Lut-lna Idctfd Lam. Mediterranean. 



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

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

 siphons. Live in fresh water. Ci/flti-xcur/ieu L., Pisidtn-ni Pf. Corbicula Miihlf. 



Fam. Cyprmidae. 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 

 siphonal openings. Cyprina ixJ//nJ/ra Lam., IsocardAa cur 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 rerrucosa L., Mediterranean. Ci/thcrca Clilonc L.. edible, 

 Mediterranean. C. Dioiu- L., Atlantic Ocean. 



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



* Fr. Levdig, "Anatomic und Entwickelung von Cyclas." Millie r'x An ////, 

 1835. 



