CONCHACEA. 



735 



and North America have yielded the remains of these singular 

 Molluscs ; but they are especially abundant in Southern Europe, 

 where they often give rise to great beds of marble (" Hippurite 

 Limestones," " Rudisten-Kalk "). In all the Rudistce. the two valves 

 of the shell are more or less conspicuously dissimilar in shape and 

 size, and often in external sculpturing or internal structure. They 

 appear to have lived in shallow water, and to have grown in great 

 beds or banks, much as Oysters do. 



Order IX. Conchacea. 



In this order, the inhalant and exhalant openings of the mantle 

 are usually prolonged into siphons ; the foot may or may not be 

 byssiferous ; and there are two adductor muscles. The shell is 

 equivalve, regular, and free ; the ligament is almost always external ; 

 the hinge has cardinal teeth, generally with lateral teeth as well ; 

 and the pallial line is usually sinuated, but may be entire. The 

 Bivalves included in this order are mostly marine, but sometimes 

 inhabit brackish or fresh waters. The principal families included 

 under this head by Fischer are the Megalodontidce, Cyprinidce, 

 Venerid(B, Cyrenidce, Ungulinidce, Unicardiidce, Tancrediidce, D071- 

 acidce, PsammobiidcB, and Solenidce. 



Family i. Megalodontidce. — In this family the shell (fig. 615) is 

 equivalve, very thick, and mostly smooth or concentrically striated, 



Fig. 6: 



-Megalodon cucullatus, from the Middle Devonian of Paffrath. (Afier Zittel.) 



the beaks being turned to the anterior side of the shell. The hinge- 

 plate is very broad, each valve carrying one or two strong cardinal 

 teeth, with feebly developed lateral teeth. The ligament is external, 

 and the posterior adductor impression is placed upon a more or less 

 prominent ridge. All the genera of this family are extinct, the prin- 

 cipal type being Megalodon itself. In this genus (fig. 615) the shell 



