PRESIDENTIAL ADDRESS. 51 



ous keels to prop it up. The more sedentary habits bring 

 less development to the foot than in the strenuous Donax. 

 Projecting as it does above the mud, no siphon is required 

 by Area. Great stacks of shells about the old aboriginal 

 camps show how important an article of food it was to the 

 blacks. 1 



This species is related to a tropical section of the genus 

 and indeed itself reaches the tropics, showing it to be a 

 northern constituent in our fauna. Though so common in 

 New South Wales, both this and Pyrazus are absent from 

 zostereta in the corresponding latitudes of South Australia. 

 At Dry Greek near Adelaide, Mr. W. Howchin finds that up 

 to a recent geological date Area trapezia was abundant, 

 but that it had suddenly and completely vanished, before 

 the time of the last deposit. 2 



It is now suggested that its extinction, and that of its 

 neighbour Pyrazus, in South Australia are due to refriger- 

 ation, and may mark a period in geological climate and 

 time subsequent to that of the Maitland raised-beach. The 

 last cold phase was reckoned by Prof. David from the 

 Kosciusko moraines to be from three to ten thousand years 

 past. 3 



The surface of the mud flats inhabited by Area was 

 exposed to the full severity of the cold when a frosty night 

 coincided with low tide. Such cold would not be fatal to 

 other species formerly associated with it in St. Vincents 

 Gulf and now surviving there, as Osbrea angasi, because 

 they descend to the depth of a few fathoms. On the 

 Pacific coast, as the cold maximum approached, the Area 

 could escape by retreating as far north as was necessary, 



1 Hedley, Proc. Linn. Soc. N.S. Wales, xxix, 1904, p. 203. 



2 Howchin, Trans. Roy. Soc. S.A.., xxxvi, 1912, p. 36. 



3 David, Helms, and Pittman, Proc. Linn. Soc. N.S. Wales, xxvi, 1901, 

 p. 64. 



