408 F. W. GAMBLE AND FREDERICK KEEBLE. 



often seawards, during the early hours of ebb tide. As a whole, 

 however, the colony remains till the flowing- tide is about to 

 cover it — a green inert mass. Then it vanishes in a few 

 minutes, to reappear next day in the same spot. 



In the spring, during April, the colonies and their con- 

 stituent patches increase so rapidly as to completely occupy 

 that belt of the shore which we call the Convoluta zone, 

 along which but few patches were previously visible. The 

 number of individuals in an average-sized colony is pro- 

 digious. Von Graff (1891) has estimated that 1 c.c. of 

 preserved Convoluta contains 28,000 specimens. As the 

 individuals are as closely aggregated in a colonial patch as 

 in a tube, we may gain a rough estimate of the numbers in 

 a given area. Allowing, then, that this number (28,000) 

 occurs on a patch of 10 c.c. in area, then 1 square metre 

 would contain 28 X 10^ animals. Since we have observed 

 colonies (one of which is drawn on PI. 30, fig. 1) distributed 

 almost evenly over an area of 200 square metres, their 

 population would not be less than 56^. 



The Convoluta zone of the Tregastel shore is a belt of 

 sand which occupies a definite position with regard to the 

 outflow of sea water due to natural drainage. 



On this shore, which presents the ordinary succession from 

 sea to land of muddy sand, fine sand, small gravel, coarse 

 gravel, and pebbles, there is a line more or less broken which 

 marks the outflow of what we may call the salt-water 

 springs. The mode of origin of the line of springs is as 

 follows: — The water brought by the high tide penetrates 

 through the pebbles, gravel, and sand to a certain depth, 

 the depth being determined by the amount of water 

 previously present and the vertical extent of permeable 

 material. As the tide ebbs, this subterranean water, as well 

 as the surface water, recedes, the rate of flow of the drainage 

 water seawards being determined by the obstructions to its 

 course. The outflow of this tidal drainage on the surface 

 of the sand commences at the line of junction of the 

 permeable with the impermeable layers. 



