INTRODUCTION. 



seas, some of the most characteristic shells are Buccinum, Tritonium, 

 Fusus, Terebratula, Rimula, &c. 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 praecisa, 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 concava. 



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, &c. 



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 Panopsea 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. 



D 



