328 



Beyond the reach of the terrigenous sediments, the floor of 

 the sea may exhibit chiefly an organic Eaeies, and this will 

 also be true of the immediate continental or island shore, 

 wherever sediments of a purely detrital character are absent. 

 It is comparatively rare that this gradation ol* fades is 

 observed iii nature. One or more fades are generally want- 

 ing, while not infrequently two or more interlock. Thus a 

 rocky cliff may descend into the sen without a beach at its 

 base: or there may be simply a bouldery beach, or only a 

 sandy or gravelly one. The landward boundary of the 

 shore may be a sand, gravel, or boulder beach, and exten- 

 sive mud-flats may stretch seaward from a shore formed 

 mainly of vegetable accumulations, such as salt marshes. 

 It will thus be seen that the shore presents the greatest 

 variety of physical features, and that hence the physical con- 

 ditions, and the organisms existing under them, must be 

 most varied. 



The shallow sea, or "Flachsee," is that portion of the 

 littoral district which is never uncovered. It is separated as 

 a distinct district bv Walther. who restricts the term lit- 

 toral to the shore zone. It is, however, so intimately con- 

 nected with the shore zone, in all its physical and bionomic 

 characteristics, that a separation is not natural. The bot- 

 tom of the shallow-sea zone of the littoral district is less 

 diversified than that of the shore zone. In its upper port ion 

 and in its shoals it may partake of the character of the 

 shore zone; but in its deeper portions the character of the 

 bottom is usually more uniform, being either rocky or, what 

 is more common, composed of fine detrital material mingled 

 with organic matter in various stages of dissolution. 

 According to the character of the bottom, plant life will 

 vary, and with it, to a greater or less extent, animal life. 



Taken as a whole, the littoral district is the most impor- 

 tant portion of the sea, both from a bionomic point of view 

 and from its bearing on palaeontology. "The littoral 

 region," says Loven,* " comprises the favoured zones of the 



Sw.ii Loven . . On Pourtalasia, a genus of Eehinoidea.— Stockholm, 1883, p. 80. 



