ASPERGILLUM. GEOLOGICAL DISTRIBUTION. 419 



nearer the larger end ; by these apertures the water is freely 

 admitted to the interior of the shell. The animals of this genus 

 are borers. Some of them live in the sand, plunged down per- 

 pendicularly for about three-fourths of their length, and sup- 

 ported by the little tubular prolongations, which are supposed 

 to be filled by fleshy filaments of the mantle. Some, again, 

 burrow in stone, others in wood, and others in thick shells. 



964. In regard to the Geological distribution of the Lamelli- 

 branchiata, it will be sufficient (as in the case of the Gasteropoda) 

 to say, that they make their appearance in the earliest fossili- 

 ferous strata ; and that, although there is always a difference 

 between the fossil and the existing species, until we compare 

 those of the comparatively recent geological periods, the same 

 genera are found to have existed in the ocean, from the most 

 ancient times to the present. It is interesting to remark, how- 

 ever, that the proportion which they bore to the Bivalves of the 

 succeeding class, was very small in the oldest fossiliferous rocks, 

 but has since been gradually reversed. 



