ASTRtEOSPONGIA. 133 



Sub-Order. — Octactinellid^. 



Genus. — AsTRiEOSPONGiA, F. Boemer. 



I860. Silur. Fauna Tennessee, p. 14. 



Syn. — Blumenbachium, F. Boemer (non Koenig) ; Astrasospongium, F. Boemer ; 

 Octacium, Schliiter. 



Generic Characters. — Sponges discoid or shallow cup-shaped in form, and 

 without stem or any surface of attachment. No special canal-system indicated in 

 the skeleton, which is composed of spicules having six rays extending in a plane 

 from a common centre, at equal angles from each other, and with a vertical axis. 

 One or both rays of the vertical axis may be reduced or not be developed. The 

 spicules are irregularly arranged, with their horizontal rays generally parallel to 

 the surface of the Sponge ; they are quite free from each other. 



Though the spicular elements of the skeleton in this genus do not appear to 

 have been organically attached together, entire Sponges are of not infrequent 

 occurrence. In these the spicules are now cemented together by the matrix, and 

 they are best preserved on the weathered surface of the Sponge. The rays in some 

 of the larger spicules show, when weathered, longitudinal open furrows, indicating 

 the presence of canals. Many of the examples of this genus are now composed of 

 carbonate of lime, and even the detached spicules met with in the rocks consist of 

 the same mineral. In some, however, the exterior is of carbonate of lime, whilst 

 the interior is a mass of chalcedonic silica, in which the spicular structure has 

 been obliterated. The frequent occurrence of these calcitic specimens has given 

 rise to the belief that the Sponges originally consisted of this mineral, and Ferd. 

 Roemer 1 supports this view by the statement that other Sponges, as Astylospongia 

 for example, occurring, with Astraospongia in the same beds in Tennessee, are com- 

 pletely silicified. But it does not thereby follow that this latter genus is a Calci- 

 sponge, since we know that many of the Silurian examples of Aulocopium, a 

 generally recognised siliceous Sponge, are partly of calcite and partly of chal- 

 cedonic silica, like some of the Tennessee specimens of Astrceospongia. The form 

 and general character of the spicules of this genus, moreover, do not indicate any 

 affinity with recognised Calcisponges, and I therefore regard the calcareous 

 specimens as replacements after silica. 



The spicules of Astrceospongia have been described by F. Roemer as consisting 

 only of six horizontal rays, and Schliiter, 2 maintaining the same opinion, has con- 



1 ' Lethaea palaeozoica ' (1880), p. 314. 



2 ' Sitzungsb. d. niederrhein. Gesellsch. in Bonn,' 1885, p. 151. 



