56 COELENTERATA—SPONGIAE CLASS I 
Tholiasterella, 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 
Asteractinella, 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. Fic. 70. 
io 79 telceawalle Tholiasterella gracilis, ; 
(Fig. (2). Thick-walled, Hinde. Carboniferous B 
depressed, bowl-shaped, — Limestone; Dalry, Ayr- 
: shire. Dermal layer with 
upper surface concave, fused stellate spicules, 
lower convex, without 
°/; (after Hinde). 

Fic. 69. Fic. 71. wees vegan 
Spiractinella Wrightii, Carter Asteractinella expansa, cha ~ 
sp. Carboniferous Limestone ; Hinde. Carboniferous Lime- Astraeospongia meniscus, Bumb,- 
Sligo, Ireland. A, Normal hex- stone; Dalry, Ayrshire. Silurian ; Tennessee. A, Sponge, in 
actin. £6, Hexactin with forked Skeletal element, °/; (after protile, 2/3; natural size. B, Upper 
rays, °/; (after Hinde). Hinde). 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-lke prominences. Common in Silurian of Tennessee and Devonian of 
the Eifel. 
According to Hinde, Tholiasterella and Asteractinella constitute a distinct 
order (Heteractinellidac), while Astraeospongia is made the type of the order 
Octactinellidae. 1 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 form a continuous framework in such a way that 
every arm of a hevactin is applied to the corresponding arm of an adjacent spicule, 
and both rays become enveloped in a common silicious covering. Loot-tuft absent. 
The Dictyonina are probably descendants of the Lyssacina (possibly from 
Protospongia- and Dictyophyton-like forms). They appear first in the Trias, and 
play a prominent réle as rock-builders in the Jurassic and Cretaceous. Their 
lattice-like skeletons are frequently replaced by calcite, or are dissolved away 
