BIVALVES. 99 



belonging to two orders the Brachiopoda and the Lamellibran- 

 chiata. The Brachiopoda took an important place among the 

 Mollusca of former ages (see vol. i. pp. 75 & 136), and were re- 

 duced, in the Miocene, to three species of Terebratula, two of 

 which (T. Buchii, Mich., and T. miocenica, Mich.) have been 

 found at La Chaux-de-Fonds. 



The Lamellibranchiata are divided according to the number 

 of muscular impressions on the inner surface of the valves, into 

 Monomyaria, with one muscle, and Dimyaria, with two muscles. 

 To the former belong the families Ostreacea, Pectinacea, and 

 Aviculacea, numerous species of which inhabited the Molassic 

 sea. 



Oysters are represented by twelve species. Among them is 

 the common oyster (Ostrea edulis, Linn.). The shells which 

 have been found at Miinsingen and St. Gall cannot be distin- 

 guished from those furnished by the present seas ; and the group 

 of the American oysters was also represented. One species (O. 

 virginica, Lam.), which now lives on the coast of Florida, inha- 

 bited the Shell-sandstone sea, and is abundant near Miinsingen 

 and in the Siggenthal ; and even the commonest species of the 

 Molasse, remarkable for its long shells weighing several pounds 

 (Ostrea crassissima, Lam.), which occurs in great beds near 

 Hiitlingen on the Belpberg and near Miinsingen, also belongs 

 to this group, which had made its appearance in the Eocene. 

 Near Hiitlingen a bed 1 J metre (or 5 feet) thick consists almost 

 exclusively of such oyster-shells. 



Pectinacea constitute a very ancient type, which peopled the 

 sea through all geological periods, and is manifested within a 

 limited number of forms in an inexhaustible multiplicity of 

 species. In the Molasse they are represented by the genera 

 Lima and Pecten. The former, which were abundant in the 

 older seas of Switzerland (see vol. i. pp. 44, 75, and 136), are 

 certainly approaching extinction, since only four species occur 

 in the Molasse, two of which (Lima inflata, Linn., sp., and L. 

 squamosa, Lam.) have continued to the present day. 



The scallops (Pecten}, on the contrary, although commencing 

 just as early, have been developed into a great variety of species 

 in all periods down to the present time. The Molassic sea con- 

 tained fourteen species, several of which (such as P. bur dig a- 



