MOLLUSCA. 123 



sents the primitive beach habitats of this region. At Hat Point, the 

 sandstone outcrops in ahnost horizontal layers to form a small cliff 

 about twenty feet high. For some distance off the Point the water 

 deepens very gradually and the bottom is strewn with large and small 

 irregular rock fragments broken from the shore by the waves. At 

 Little Oak Point the conditions are similar except there is no cliff and 

 the outcrop is smaller. 



In most places the sand has been washed up over the rock outcrops 

 and has covered them entirely, but at Hat Point the overhanging cliff 

 still outcrops some distance in from shore and forms damp caves (habitat 

 27). In the former case, unprotected, sand beaches (habitat 5) are 

 produced. 



TwQ kinds of currents, the off-shore and the littoral, work on the ex- 

 posed, sandy shores. The off-shore currents build points or sand hooks, 

 while the littoral currents make barrier beaches parallel with the shore. 

 The former are usually formed in partially protected places, the latter 

 on the side from which the currents come. Behind these points and 

 bars, the water is more nearly calm and the sand is not disturbed so 

 that another habitat, that of the protected, sandy shores (habitat 7), 

 is formed. This kind of a habitat is also found on the protected side 

 of projections not built of sand. 



The barrier beaches often become connected at both ends with the 

 shore, forming shallow lagoons or sand pools (habitat 6). New beaches 

 are built up farther out and the sand dunes are blown up around these 

 lagoons, the larger of which become long, shallow lakes (habitat 9), 

 or swamps (habitats 18, 19). In a similar manner the free ends of the 

 sand spits produced by the off-shore currents swing in towards shore 

 and form, at first partially land-locked coA'es (habitat 8), and finally 

 lakes (habitat 10), which are usually deeper and more nearly circular 

 than those produced by the barrier beaches. Swamps of a similar 

 shape may also be produced in this manner, if the vegetation in the cove 

 forms a deposit of muck fast enough to fill the cove before it becomes 

 a lake. 



The sand beaches along any of these places form another habitat 

 (habitat 25), and the wind, blowing back the sand from these beaches, 

 builds up rows of sand dunes (habitats 28, 29). with swamps (habitats 

 18, 19) or even long, shallow lakes (habitat 9). similar in shape to those 

 formed from the sand pools, between them. 



The vegetation in all of these lakes soon starts depositing material 

 which converts them first into perennial (habitat 19), and then into 

 annual swamps (habitat 18). and finally fills them entirely, forming 

 alluvial flats (habitat 26). This filling in takes place mainly around 

 the edge where the water is shallow, so that all except the very young- 



