MOLLTJSCA OP LOWER CANADA. 59 



to the north it is noticeable that several shells, mostly littoral 

 species, occur on both the Pacific and Atlantic shores. Mocliola 

 modiolus, Crenella discrepans, Tricbo.tro.pis borealis, and Bela tur- 

 ricula, inhabit Oregon, north-eastern America, and northern 

 Europe. Referring again to Mr. Carpenter's Report we see that 

 sixteen species of Arctic mollusca inhabit both the Atlantic and 

 Pacific. These are : — 



Rhynchonella psittacea, Gmel. Trichotropis borealis, Brod. & Sow. 



Mya arenaria, Linn. Admete viridula, Fabr. 



Machoera costata, Say. Scalaria Grcenlandica, Ghemn. 



Tellina solidula. (T. fusca, Say.) Natica clausa, Brod. & Sow. 



Mactra ovalis, Gould. Purpura lapillus, Linn. 



Mytilus edulis, Linn. Fusus Islandicus, Linn. 

 Anomia patellifornis, Linn. " antiquus, Linn. 



Margarita arctica, Leach. Trophon clathratus, Linn. 

 " helicina, Mole. 



The majority of these are species of considerable geographical 

 distribution ; all but two (Machoera costata and Mactra ovalis) also 

 inhabit northern Europe. The Tellina nasuta of Conrad, from Ore- 

 gon, may be a geographical variety of the Tellina proxima of the 

 eastern coast. In like manner Turritella Eschrichtii may be 

 Scalaria borealis, and Littorina Sitchana of Philippi (also from 

 Oregon) may be only a variety of Littorina p«tula. We have seen 

 that eleven of the Lower Canadian fresh-water shells also inhabit 

 the west coast of iNorth America. Yet the grand chain of the 

 Rocky Mountains intervenes, presenting, according to the views 

 of most naturalists, an impassable barrier to migration. How 

 then can we account for this apparent anomaly? Admitting 

 that during the post-pliocene period, a great, but gradual depress- 

 ion of the land took place on this continent, do we not begin to see 

 our way a little more clearly ? When the mountain tops alone 

 were left uncovered by the ocean, these snails, for instance, 

 could only remain on, or near, the dry land, and when the land 

 re-assumed its present shape and general physical condition, 

 the whole area would be peopled, in part, from these sources. 

 For supposing these creatures confined by the above mentioned 

 causes to what are now the peaks of the Rocky Mountains, it is 

 not difficult to conceive, that on the gradual re-elevation of 

 the land, these molluscs could extend in both an easterly and 

 westerly direction. Whether the theory I have advanced be true, 

 or whether it is more likely that such sluggish creatures as fresh- 



