424 PROCEEDINGS OF SECTION C. 



The stratigraphy of the Archpeocyathinse horizon is engaging the 

 attention of that indefatigable South AustraUan geologist, Mr. W. 

 Howchin, F.G.S. ; but a few words as to the extent of the " Cambrian 

 Reef " may not be out of place. 



The magnitude of this old reef will be appreciated when it is stated 

 that at the Ajax Mine — .lO miles north of Beltana — .its width (with a 

 dip of 68°) approaches 300yds. There is little doubt that as the local 

 geology of South Australia is worked out more and more localities will 

 be discovered at intermediate points over this 400-mile belt, and that, 

 in addition to the most magnificent modern reef (the Great Barrier), 

 Australia will be found to possess the greatest paleozoic reef in the world. 



A factor rendering the South Australian specimens of especial 

 value is that most of the Ajax specimens have undergone silicification 

 to a most wonderful extent. By acting on the matrix with hydro- 

 chloric acid, and carefully coating delicate structures with paraffin as 

 they appear, it is possible to etch to a depth of four or five millimetres 

 without destroying the meshwork of which walls, septa, and tabulae are 

 composed. So far as I am aware, no other Archa^ocyathime specimens 

 admit of this treatment. There is, however, little doubt that the 

 original skeleton was calcareous and not siliceous. The organisms 

 from Sellick's Hill consist entirely of calcareous matter ; while the 

 Ajax specimens are sometimes only replaced by silica in the peripheral 

 portion of the matrix in which they are preserved. Thus, in some 

 instances, one side of a cup will resist the acid while the other is entirely 

 dissolved. 



NOMENCLATURE. 



In the following descriptions the terms septa, calyx, tabulae will 

 be used to denote structures similar in appearance to, but probably not 

 homologous with, corresponding structures in the corals. The outer 

 wall and inner wall do not resemble very closely the thecal elements of 

 the corals. In the flabellate genera the lower proximal wall (from 

 which proceed the rhizoids) and upper distal wall are analogous to the 

 outer and inner walls respectively of the turbinate and bell-shaped 

 genera. The term septum (denoting simply a partition) can still be 

 used for the vertical plates connecting the two walls at regular and 

 frequent intervals in the flabellate (or expanded) genera. The rec- 

 tangular chambers bounded by the adjoining septa and the walls will 

 be termed " loculi." The latter will be cuboid (isometric) or brick- 

 shaped, according as the tabulae are numerous or remote. 



The above terms are illustrated and more fully defined in the 

 sequel. 



MORPHOLOGY. 



The original Archaeocyathus of Billings — on which the family was 

 founded — -is described as follows : — ■ 



" Turbinate, simple or aggregate, cup deep. Internal structure, 

 so far as can be made out, consists of an inner wall, constituting the 

 inner surface of the cup, and an external wall, or epitheca, enveloping 

 the whole. Between the two walls there are numerous radiating septa, 

 the interseptal spaces being filled with poriferous or septal tissue." 



