INTRODUCTION. sii 
seas, some of the most characteristic shells are Buccinum, Tritonium, 
Fusus, Terebratula, Rimula, &. Around Cape Horn are shells of the 
same types, so closely allied that they have not yet been separated as 
distinct genera, though peculiar in many important respects. But this 
resemblance does not descend to species. In the first case, however, 
not only have we the same genera, but the species seem to repeat each 
other: so that species brought from great distances east or west, are 
scarcely to be distinguished upon comparison. As examples in illus- 
tration, we may place against each other the following species from 
Oregon and from the Eastern States : 
Mya precisa, Mya truncata. 
Osteodesma bracteatum, Osteodesma hyalina. 
Cardita ventricosa, Cardita borealis. 
Cardium blandum, Cardium Icelandicum. 
Venus calcarea, Venus mercenaria. 
Alasmodonta falcata, Alasmodonta arcuata. 
Helix Vancouverensis, Helix concaya. 
Helix loricata, Helix inflecta. 
Helix germana, Helix fraterna. 
Planorbis vermicularis, Planorbis deflectus. 
Planorbis opercularis, Planorbis exacutus. 
Lacuna carinata, Lacuna vincta. 
Natica Lewisii, Natica heros. 
Trichotropis cancellata, Trichotropis borealis. 
Fusus fidicula, Fusus turricula. 
Lottia pintadina, Lottia testudinalis, &e. 
Mingled with these are others very different in type, which mark 
the two localities as constituting very different zoological regions. 
Where, for instance, have we the analogues of Panop#a generosa, 
Lutraria ventricosa, Triton Oregonense, on the one hand, and of 
Mactra gigantea, Fusus decemcostatus and Icelandicus, Pyrula cana- 
liculata and carica, Pandora trilineata, &c., on the other? The same 
comparison holds good between the shells of the Gulf of California and 
the Gulf of Mexico. 
