130 BRITISH SEA-WEEDS. 



on the subject, and much observation and study will 

 have to be expended on the genus before the species can 

 be satisfactorily determined. Turning for a moment 

 from the consideration of the details of genera and spe- 

 cies to glance at the gigantic processes which are in con- 

 stant progress around us, it is interesting to note not 

 only how various agencies are employed each for the 

 attainment of its required end, but even how the same 

 object is occasionally accomplished by different means. 

 The vast chalk formation which plays so important a 

 part in the economy of this country was all held in so- 

 lution in the sea which formerly covered the space now 

 occupied by these islands. It was collected and depo- 

 sited by myriads of mollusks, coral-polyps, sponges, and 

 sea- weeds, — the latter of kindred nature and structure to 

 those of the Order Corallinacece. And if we examine 

 the work while it is in progress we shall find that al- 

 though the agencies are so different, and belong half to 

 the Animal and half to the Vegetable Kingdom, the 

 modes of action are very similar. Thus the animal 

 coral-polyps and the vegetable Melobesise, at least cer- 

 tain species, spread themselves over vast tracts of the 

 bottom of the sea, and new coral is formed, or new 

 fronds grow in constant succession, layer above layer : 

 that which is beneath being left to die, decay, and de- 

 posit its calcareous remains to form chalk for future 

 generations. 



Melobesia calcarea. The chalk Melobesia. 



Fronds formed of uniform, globular cells, free, coral-like ; 

 branches irregularly divided, spreading, tapering to a blunt 

 point ; branchlets simple or forked. 



This species is very common, particularly on our 



