102 MIOCENE FAUNA. 



Labordii, Bart., and uniradiata, Br.), the nearest allies of which 

 live in tropical seas ; the Tellinte also occur in the greatest 

 abundance and variety in the torrid zone, and possess only a few 

 insignificant species in northern regions. Of the twelve species 

 found in the Swiss Molasse, three (Tellina senegalensis, Hani., 

 T. lacunosa, Chenm., and T. crassa, Gmel.) are West-African 

 forms, although Tellina crassa occurs in the North Sea and, 

 with six other species, in the Mediterranean. It is worthy of 

 notice that the Eocene species are of tropical and Australian 

 types, but that half of the Miocene species are still continued in 

 European seas. 



The Corbulacea, in Tugonia anatina, afford a West-African 

 type, and in Corbula they show a genus which previously occurred 

 as early as the Carboniferous epoch. Of the latter the Molassic 

 sea was inhabited by three still- existing species, one of which (C. 

 carinata, Duj.) is now found in tropical Asia, the second (C. re- 

 voluta, Broc.) occurs in the Mediterranean, and the third (C. 

 gibba, Ol.) on almost all European coasts. 



The Pholadomya were represented in the Jurassic sea (see 

 vol. i. p. 136) by numerous forms ; at the commencement of the 

 Tertiary period they had already become rare, and at present 

 only a single West-Indian species remains of the genus. The 

 two species of the Molasse ( Pholadomya helvetica, May., and P. 

 arcuata., Lam., from St. Gall and Lucerne) are extinct. On the 

 other hand, the Amphidesmq-grou]) is represented by three 

 European species (A. Syndosmyd) ; and the Pandorida has also 

 three species in Europe. 



The Mactrina have been distributed over all seas from very 

 ancient times. Twenty-five species are known from the Molasse, 

 most of them belonging to the genera Mactra and Lutraria, 

 forms which bore into the sandy bottom of the sea. The Mio- 

 cene species no doubt had the same mode of life ; and to them is 

 probably to be ascribed the production of those remarkable spi- 

 rally twisted structures which have been found in various parts 

 of the Molasse. These are rod-like bodies, about as thick as 

 one's finger, on the sides of which are seated spirally twisted 

 branches of equal thickness. Probably several animals lived 

 together ; first of all they would dig a hole perpendicularly into 

 the sand, and then, starting from this aperture, make several 



