•26 



THE GEOLOGIST. 



LiGH. 5. Recent Mussel (MjtiliM edulis) moored by its bysaus. 



the means of respiration and the source of food. These Kve erect 

 in the oose, others with plated gills, like the oyster or the spondylus, 

 rest on one of their sides, while some like the mussel or the 

 terehratula, are moored to the bottom by a byssus or set of natural 

 cords. Each, however, has its normal position, and each thus by 

 its position in the mud or rock, tells us, more or less distinctly, 

 where it died, whether in its native home or at some distant spot. 

 We know too by their being disposed throughout the stratum, 

 without regard to special levels, that these molluscs lived and died 

 naturally at that very place, that the sediments growing higher and 

 higher, — film after film piled over other, — the young fry settled 



liGiT. G. Shells at all levels in the strata of mud, shewing the gradual accumulation of sediment 

 and the natural interment of the molluscs. 



above the gi-aves of their progenitors, and like them, in course of 

 time were enveloped by the accumulating mud to be again sur- 

 mounted by younger colonies who in their turn again succumbed. 



But where all on one line in their noraial states, the fossil 

 shells make one thin calcareous streak in the consolidated mud. 



