718 CLASS XII. 



genus the only species hitherto known, Glycimeris siliqua, is 

 found very abundantly on the great bank of Newfoundland. More 

 numerous, on the other hand, are the forms which occur exclu- 

 sively in warm seas or in the southern hemisphere, or of which 

 one or only a few species are met with in the Mediterranean, 

 or the Red Sea. We mention here the genera Crassatella, Tri- 

 dacna, Hippopus, Malleus, Avicula, Meleagrina, Spondylus (and 

 the genus Plicatula united with it), Vulsella, Placuna. The fre- 

 quent occurrence of well-preserved remains of this class in differ- 

 ent mountain-strata affords an assistance of the highest interest to 

 the investigations of geologists towards a true knowledge and 

 distinction of those strata. Of some genera the extinct species are 

 much more numerous than those now living ; some genera which 

 formerly peopled the sea have entirely disappeared in the present 

 period of the history of the earth. It is remarkable that of the 

 genera now living and which also count extinct species, whenever 

 these last are very numerous, by far the most of the living species are 

 now met with either exclusively, or principally, in the Indian ocean 

 and in the southern Pacific. Of the genus Trigonia, a single living 

 species alone is now known, and this is found in the South Sea at 

 New Holland, whilst numerous fossil species belong to it, especially 

 from the Jura- and chalk-formations. The genera Spondylus, Lima, 

 Avicula, Crassatella, Area and others, tend also to prove the same. 



Regarding the fresh-water conchifers, we would call attention 

 to the great number of species of the genus Umo, which occur in 

 the western hemisphere, especially in North America. 



The Brachiopoda in their geological and geographical distribu- 

 tion offer much that is peculiar. Of Orbicula and Terebratula 

 species occur both in the north and in the south ; species are known 

 from the Indian ocean and from the Mediterranean, from the South 

 Sea on the west coast of America, and from the North Sea. At 

 the same time of these genera the species in the southern seas are 

 more numerous, and the few species of Lingula are all from the 

 southern hemisphere. Extinct species of Terebratula are uncom- 

 monly numerous, and occur in very old as well as in more recent 

 formations. Thus the Brachiopoda make one of the few forms 

 which are restricted to no limited period of the history of the 

 earth, and have survived its various catastrophes. 



