SAXICAVA. 139 



we have seen when treating of Pholas, expressly asserts 

 that the Saxicava: bore by rasping, effected by means of 

 siliceous particles contained in the anterior part of the 

 mantle. Mr. Couch entertains a similar view. We have 

 not been able to satisfy ourselves of the presence of such 

 particles, though inclined to regard such a view with favour, 

 as in this case the surface of the shell does not seem devised 

 for rasping as is that of the shells of the last tribe. 



Great interest attaches to the British species of this 

 genus in a geological point of view ; one, if not both of 

 them, OAving a wide distribution, in the present epoch, to 

 events which occurred in pre-adamite ages. The researches 

 of geologists have made known to us, that, previous to the 

 present state of things, within the area of our islands, there 

 existed climatal conditions much more severe than those 

 which now prevail, — that, in fact, the climate of Green- 

 land, and the fauna and flora of the regions in which that 

 climate is now met with, then extended over the greater 

 part of Europe and Northern Asia, having its southern 

 bounds somewhere in a line with the southernmost part of 

 the British Islands as they are now constituted. At that 

 time, however, the greater part of our country was under 

 water, and represented by ridges of land and small islands, 

 rising in the midst of an icy sea. During this chilly epoch 

 the Saxicava extended their range almost round the whole 

 of the northern hemisphere, and, when the bed of the 

 glacial ocean was upheaved, — as geological research proves 

 to have been the case, previous to the present arrangements 

 of our region, and preparatory to a more genial assem" 

 blage of conditions, — the shells of these mollusks were pre- 

 served in the raised sea-beds, and are found in them now, 

 even at elevations of several hundred feet about the level of 

 the present sea. Thus we find them in Sweden, where 



