PORIFERA. CALCAREA 45 



CLASS III. CALCAREA 



(CaLCISPONGIjE) 



The skeleton consists of spicules composed of carbonate 

 of lime in the condition of calcite. The spicules are 

 usually much smaller and less varied in form (fig. 9, j) 

 than those of the siliceous sponges, and cannot be separated 

 into megascleres and microscleres. There are three kinds, 

 the simple uniaxial, the three-rayed, and the four-rayed ; 

 they are sometimes fused with one another, but often are 

 either arranged close together so as to form fibres, or are 

 loosely distributed. Spongin is never present. The earliest 

 British forms of the Calcarea occur in the Carboniferous 

 rocks of Fifeshire. 



Peronidella. Cylindrical, simple or branched ; central cavity 

 tubular and extending from the summit to the base of the sponge. 

 Walls thick and with no definite canals, but having irregular spaces 

 between the spicular fibres. Spicules three- or four-rayed, forming 

 anastomosing fibres. Carboniferous (possibly also Devonian) to 

 Cretaceous ; most abundant in the Jurassic and Cretaceous. Ex. 

 P. pistilliformis, Great Oolite and Cornbrash. 



Corynella. Form similar to Peronidella. Radial excurrent 

 canals open into the central cavity, which often does not extend to 

 the base of the sponge, but is continued downwards by vertical 

 canals. Incurrent canals fine, directed obliquely downward. Osculum 

 usually with radial furrows. Jurassic and Cretaceous (? Trias). Ex. 

 C. foraminosa, Lower Greensand. 



Holcospongia. Simple or compound :• individuals usually 

 spherical, hemispherical, or club-shaped ; their summits rounded, 

 with a central area in which a number of excurrent canals open, 

 and from which furrows extend down the sides of the sponge. 

 Spicules large and three-rayed, and some also filiform ; and a surface 

 layer of three-rayed spicules, of various sizes, felted together. 

 Inferior Oolite to Cretaceous. Ex. H. polita, Corallian. 



