The Bivalves
(_Lamellibranchs_) and the Univalves (_Gasteropods_) are exceedingly
numerous, and almost all the principal existing genera are now
represented; though less than five percent of the Eocene _species_
are identical with those now living. It is difficult to make any
selection from the many Bivalves which are known in deposits of
this age; but species of _Cardita, Crassatella, Leda, Cyrena, Mactra,
Cardium, Psammobia_, &c., may be mentioned as very characteristic.
The _Caradita planicosta_ here figured (fig. 216) is not only very
abundant in the Middle Eocene, but is very widely distributed,
ranging from Europe to the Pacific coast of North America. The
_Univalves_ of the Eocene are extremely numerous, and generally
beautifully preserved. The majority of them belong to that great
section of the _Gasteropods_ in which the mouth of the shell
is notched or produced into a canal (when the shell is said to be
"siphonostomatous")--this section including the carnivorous and
most highly-organized groups of the class. Not only is this the
case, but a large number of the Eocene Univalves belong to types
which now attain their maximum of development in the warmer regions
of the globe. Thus we find numerous species of Cones (_Conus_),
Volutes (_Voluta_), Cowries (_Cyproea_, fig. 218), Olives and
Rice-shells (_Oliva_), Mitre-shells (_Mitra_), Trumpet-shells
(_Triton_), Auger-shells (_Terebra_), and Fig-shells (_Pyrula_).
Pages:
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439