Handbook of Paleontology 117 



species of a type present one is apt to be the most 

 abundant and may be the dominant species. In one 

 species the individuals may be small but very numer- 

 ous, while larger, more conspicuous forms are present 

 in relatively small numbers. There is a pronounced 

 variation in the extent to which the different phyla 

 are represented in the associations. The mollusks are 

 found on rocks and in tide pools as well as on sand. 

 In the former situations one finds the mollusks fewer 

 and of the creeping type, and therefore the gastropods 

 predominate; in sand, however, mollusks are numer- 

 ous, mostly of the burrowing type, and here the lamel- 

 libranchs or bivalves predominate. In the rocky pools 

 the coelenterates, such as the hydroids and sea anemo- 

 nes are well represented. In all the associations, it 

 has been noted that a balance is maintained between 

 vegetable species, predatory species and parasites. 

 Otherwise, changes in the association would result. 



The three main types of shore recognized are (1) 

 rock, (2) sand, (3) mud. There are a number of sub- 

 types within each type and between the different 

 types. Some in studying beaches recognize a sub- 

 merged zone, the lower beach and the upper beach; 

 others have divided both the sandy and muddy beaches 

 into a number of zones. Rocky shores are likewise 

 so treated. The area between high and low tides can 

 usually be split up into a number of life zones which 

 are characterized by certain assemblages of forms. These 

 forms may be confined to that one zone or if present 

 in other zones may in the former be predominant in 

 numbers or have some well-marked character which 

 gives a distinctive, readily recognized, appearance or 

 "fades" to that particular horizon. Associations of the 



