170 On the Sections of the Delta of the Yarra, 



them by wading near the mouth of the river on the Sand- 

 ridge shore. 



Rev. J. E. Tenison- Woods, in his " Census of Tasmanian 

 Shells/' quotes A. trapezium as "frequent at Tamar Heads 

 at low water." But on communicating with my friend, Lieut. 

 Beddome, of Hobart, a high authority on Tasmanian shells, 

 he forwards to me specimens of A. fasciata as being the 

 species abundant among the rocks at Tamar Heads. I have 

 often found A. fasciata in holes in the rocks just below low 

 water at Williamstown, Cheltenham, &c. I think that A. 

 trapezium occupies muddy bottoms, in comparatively deep 

 water. 



With regard to the present habitat of the mud oyster, Mr. 

 Bracebridge Wilson writes me that he has dredged them 

 alive in Corio Bay. I have it also from Ernest Myers, a 

 Sandridge fisherman, that they are occasionally to be found 

 in Port Phillip. Mr. Wilson says that the dead shells are 

 abundant in the South Channel, and adds that a fisherman, 

 named Mentiplay, used to dredge live oysters for sale from 

 that locality. 



The silt is very fine, and is from pressure compact and 

 impervious to water. That this property has been of service 

 in the preservation of the shells is seen from the effect pro- 

 duced upon them when exposed to the action of the river 

 water. I have never found any concretions in the shell- 

 layer in the canal itself. They are frequently brought up 

 in dredging from the river. Mr. Da vies obtained them i7i 

 situ from the shell bed in the slip mentioned as cut some 

 500 feet from the east end of the cutting. No water came 

 out of the silt until the shell band was reached ; then water 

 oozed out in quantity. I have no doubt that this was 

 derived horizontally from the river, and that the access in 

 this way of river water has produced these calcareous con- 

 cretions. The smaller shells, where acted upon by water, 

 have yielded up to it their lime, and this, carried in solution, 

 has penetrated between the grains of silt, uniting them into 

 an excellent " cement." The heart of the concretions is still 

 dark silt. All stages of the process are visible from the 

 unaltered shells, still distinct in outline, though united into a 

 cluster by a hard cement, until we reach the nodular con- 

 cretion of indefinite shape. In the case of the thinner shells 

 only has the process been completed. The thick dense shells 

 of the areas have in very few cases yet succumbed, but they 

 manifest signs of the initial stages. 



