718 CLASS XII. 
genus the only species hitherto known, Glyctmeris siliqua, 1s 
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, Arca 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 Unio, 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. 
