Dr. R. B. Roll— On Fossil Sponges. 309 



After the rivers had thus coursed across the broad deposits of 

 sand, which are inferred to have covered so large a part of Sweden, 

 a movement of subsidence ensued, and the Malar basin became con- 

 verted into a shallow sea. During this period of depression, the 

 fine sand, which was unprotected by the coarse gravel and shingle 

 of the river-beds, was washed away, and thus the asar were formed. 

 As the downward movement continued, stratified clay, with Arctic 

 shells, was deposited all round the asar. By and by the climate be- 

 came milder, and the present Baltic fauna succeeded the Arctic one. 

 The land began to rise again, and the asar, being brought nearer to 

 the surface of the sea, formed long shoals or banks, exposed to the 

 action of the waves, which reassorted their outer portions, adding 

 new layers of sand, interstratified with shell marl. The asar of the 

 lowlands thus became cloaked in a post-glacial covering ; to which, 

 and not to the asar, properly belong those shells of recent Baltic 

 species, which are got near the surface of the ridges. 



Mr. Tornebohm's objections to the marine origin of the asar are : 

 the great length of these remarkable ridges ; their common occur- 

 rence in narrow valleys ; their river-like ramifications ; and the fact 

 that they are met with at greater elevations than any undoubted 

 marine drift-deposits. The highest shell-deposits yet seen in Scan- 

 dinavia are situated not more than 600 feet above the sea-level, and 

 he thinks it unlikely that the sea ever reached a greater height upon 

 the land. The asar, however, are not limited to that height, for he 

 has traced well-marked asar up to 1000 feet, and in the mountain- 

 ous valleys of the north even up to elevations of over 2000 feet. Mr, 

 Tornebohm does not disguise from himself that it is difficult to 

 understand how the denuding force acted, which is supposed to have 

 swept away the fine sand and left the coarse gravel. If marine 

 denudation cleared away the sand and thus gave rise to the asar of the 

 lowlands, how would he explain the formation of those asar which 

 reach to heights above 600 feet — that being, as he thinks, the 

 greatest elevation in Sweden attained by the sea in late glacial or 

 post-glacial times ? 



IV. — Notes on Fossil Sponges. 

 By Hakvey B. Holl, M.D., F.G.S. 



I. Introduction. — It has been said, with some degree of truth, that 

 our knowledge of the lower forms of life has advanced pari passu 

 with the improvements in the construction of microscopes ; and no 

 doubt very considerable progress has been made in some depart- 

 ments of investigation within the last few years. In the vegetable 

 kingdom more especially has this been the case ; and with respect 

 to animals, the structure of the Protozoa has been especially eluci- 

 dated by able observers, the SpongiadE© chiefly by the labours of 

 Dr. Bowerbank. 



Among the lower forms of extinct life, progress has necessarily 

 been slower, especially as regards the sponges, for although new 

 forms have been described from time to time, the group collectively 



