1 66 



PORIFERA. 



found in water of moderate depth. As regards the distribution of 

 the order in time, one of the most ancient representatives of the 



group would appear, from the 

 researches of Dr Hinde, to be 

 the genus Archceoscyphia (based 

 upon the Archceocyathus Min- 

 ganensis of Billings), which oc- 

 curs in the Upper Cambrian 

 rocks (Calciferous series) of 

 Canada, along with allied types 

 such as Calathium. Closely 

 related forms are found at a 

 corresponding geological hori- 

 zon (the Durness Limestone) 

 in Britain. The skeleton in 

 Archceoscyphia Minganensis (fig. 

 56) has the form of a curved 

 funnel attached by its pointed 

 base, and with a deep central 

 cavity or " cloaca " apparently 

 without an internal wall. The constituents of the skeleton have a 

 radial disposition, and distinct spicules, usually four-armed, and inter- 



Fig- 55- — A, Part of the spicular network of As- 

 tylospongia, from the Silurian of Gotland, enlarged 

 60 times ; b, Rhizomorine spicule of Scliscothon 

 Mantelli, from the Cretaceous rocks, enlarged 60 

 times. (After Hinde.) 



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Fig. 56. — Restoration of the lower part ot 

 Archceoscyphia Minganensis. a, The pores 

 of the inner wall of the cup. Ordovician. 

 (After Billings.; 







Fig- 57- ■ — Section of Astylospongia 

 prcenwrsa, Silurian, Tennessee. (After 

 Roemer. ) 



linked by their subdivided ends, are present. The siliceous nature 

 and spicular structure of the skeleton in Archceoscyphia Minganensis 

 render it clear that we have to deal here with a genuine Sponge. 



