56 



COELENTERATA SPONGIAE 



CLASS I 



ThlM>rella, Hinde (Fig. 70), from the Carboniferous, has thin walls com- 

 posed of a layer of robust, irregularly amalgamated hexactins. As a rule, two 

 of the rays lying in the same plane divide dichotomously from the nodes 

 outward, so as to produce a six-armed instead of a four-armed cross. In 

 Jxtri-<ii-tiii<-!la, Hinde (Fig. 

 71), all of the rays lying 

 in the same plane divide 

 in two or more branches, 

 thus giving rise to many- 

 rayed, extremely diverse, 

 stellate, and corolla -like 

 bodies. Carboniferous ; 

 Ayrshire. 



Astraeospongia, Roem. 



(V\(T 7'^} TVnVk walWI 77/"/;.^,v// gruciHs, 



6Cl ' Hinde. Carboniferous 



depressed, bowl - shaped, Limestone ; Dairy, Ayr- 



. shire. Dermal layer wil.li 



upper surface concave, fused stellate spimies, 

 lower convex, without B /i <** Hinde). 



FIG. 70. 



FIG. 69. 



ll'rightii, Carter 

 sp. Carboniferous Limestone ; 

 Sliu'o, Ireland. A, Normal hex- 

 actin. B, Hexactin with forked 

 rays, 5/i (after Hinde). 



FIG. 71. 



Asteractinella expansa, 

 Hinde. Carboniferous Lime- 

 stone ; Dairy, Ayrshire. 

 Skeletal element, 5/i (after 

 Hinde). 



FIG. 72 



Astraeospongia meniscus, Bumb. 

 Silurian; Tennessee. A, Sponge, in 

 profile, -/3 natural size. IS, Upper 

 surface of same. 



traces of attachment. Skeleton composed of relatively large, homogeneous, 

 uncemented cruciform spicules ; six of the rays are disposed in the same plane, 

 while the two rays projected at right angles to these are reduced to short, 

 button-like prominences. Common in Silurian of Tennessee and Devonian of 

 the Eifel. 



According to Hinde, ThoUasterella and Asteractinella constitute a distinct 

 order (Heteractinettidae), while Astraeospongia is made the type of the order 

 Octaclinellidae. I should prefer, however, to regard these two groups as 

 aberrant Hexactinellids, in which supernumerary rays are produced by 

 branching. 



Sub-Order B. DICTYONINA. Zittel. 



Skeletal spicules cemented to fwm a continuous framework in such a way that 

 every arm of a hexactin is applied to the cm-responding arm of an adjacent spicule, 

 and both rays become enveloped in a common silicious covering. Hoot-tuft absent. 



The Dictyonina 'are probably descendants of the Lyssacina (possibly from 

 Protospongia- arid Dictyophyton-like forms). They appear first in the Trias, and 

 play a prommen4r0/<2 as rock-builders in the Jurassic and Cretaceous. Their 

 lattice-like ske4e"ton$* are frequently replaced by calcite, or are dissolved away 



