Displayed in the Fisher^inans Bend Cutting. 173' 



gracilis) var., N. circumsuta, Melosira Borreri, Hyalodiscus 

 Californicus, Actinocyclus BarJdyi, Navicula Yarrensis, 

 N. Smithii, Surirella craticula (so cPvlled, = Navicula sp.), 

 Epithemia musculus. 



As this is a record of facts, while I have endeavoured to 

 show their immediate bearing and connection, I have ab- 

 stained from any wide speculation as to the past history of 

 the Lower Yarra. I do not think that the depth of the silt 

 is at all an accurate measure of the time that has elapsed 

 since the areas and oysters lived. Soundings made in Hob- 

 son's Bay by Mr. Moseley in 1867, 1869, 1871, and 1878, and 

 by Captain Stanley in 1875, showed that over the western 

 part of the Bay there had been in the eleven years a general 

 deposit of 12 to 86 inches of silt. The area within a radius 

 of a quarter of a mile from Williamstown Pier had, however, 

 not shallowed at all in the period. With variations from an 

 increase of nil to an increase of 36 inches in eleven years 

 we have no grounds for a choice of rate of deposition, nor 

 reason for striking the average. It is advisable, then, to 

 postpone any extensive generalisations until other parts of 

 the delta have yielded further evidence. It is to be hoped 

 that opportunities will be seized as they occur of recording 

 such evidence. 



Art. XIV. — On the Sound Organs of the Green Cicada. 

 Cyclochila Australasice, Donovan sp. 



By a. H. S. Lucas, M.A., B.Sc, F.G.S. 



[Eead 14th October, 1886.] 



Of all insects the cicadas have perhaps for the longest 

 period attracted the curious interest of men of all nations 

 and on all the continents. It is then not a little singular 

 that there should be any disagreement amongst zoologists as 

 to the precise manner in which these assiduous musicians 



