78 Remarks on Mollusca and Shells. 



isothermal latitude. In the latter case we may have analogous 

 genera. Along our northern seas, some of the most charac- 

 teristic shells are, Buccinum. Tritonum, Fusus, Terebratida, 

 Iluiiula, &c. Around Cape Horn are shells of the same types, 

 so closely allied that they have not yet been separated as dis- 

 tinct 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 distin- 

 guished upon comparison. As examples in illustration, 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 ferox. 



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 zoologi- 

 cal regions. Where, for instance, have we the analogues of 

 Parnopcea generosa, Lutraria ventricosa, Triton oregonense, 

 on the one hand, and of Mactra gigantea, Fusus decemcosta- 

 tus and Icelandicus, Pyrula canaliculata and carica^ Pando- 

 ra trilineata, &c, on the other \ The same comparison holds 

 good between the shells of the Gulf of California and the 

 Gulf of Mexico. 



From a consideration of the land-shells collected on the 

 Pacific islands, it seems possible to draw some fair inferences 



