POEIFEKA (sponges). 
35 
Euretidte or Craticularida3 (Jurassic to Eecent) : mostly 
cup-shaped or funnel-shaped ; spicular nodes simple and 
imperforate ; canal openings large, simple, ending blindly in 
the skeleton. Mellitionidae (Cretaceous, Eocene). Coscino- 
poridfE (Cretaceous to Eecent) : in addition to the cup and 
funnel shape, many have thin walls folded into a series ot 
flanges, e.g. in the Cretaceous Gucttardia ; surface canal- 
openings small, usually arranged in quincunx. Stauro- 
dermidie (Jurassic and Cretaceous): usually funnel-shaped 
or cylindrical, with an irregular skeletal mesh and a 
definite dermal layer in which are large cross-shaped spicules, 
(kallodictyonidie (Cretaceous). Ventriculitidse (Jurassic and 
Cretaceous) : mostly funnel-shaped, with a thin wall thrown 
into vertical folds which are usually arranged radially 
(Fig. 10) ; spicular nodes hollow or lantern-shaped (Fig. 9 a ) ; 
the base of the sponge has root-lilve extensions of spicular 
fibres. Coeloptychidae (Cretaceous) : mushroom-shaped with 
a thin wall thrown into radial folds and enclosed in a 
perforate dermal layer ; canal-openings in rows on the ridges 
of the under surface ; lantern nodes. MjeaiidrospongidcC 
(Cretaceous to Recent) : pear-shaped, sack-shaped, or nodose 
masses, with a thin wall thrown into numerous folds, which 
join one another irregularly and are often partly or wholly 
enclosed by a fine spicular membrane, e.g. Gamer ospoTigia, 
Cystospongia, and Plocoscypliia. 
Class III.— DEMOSPONGIAE. 
' SRicispongiae without triaxon spicules. These are the 
commonest sponges of the present day, most familiar in the 
freshwater sponge and the bath-sponge, but found in all 
waters in the most varied suiTOundings. Palaeontologists, 
however, are only concerned with those that retain siliceous 
spicules. According to the form of those spicules they may 
be divided, somewhat artificially, into two Sub-Classes. 
Sub-Class I. — tethactinelltda. Demospongiae 
typically with tetraxon spicules (Fig. 11). 
(9rder I. — Choristida (to which the term Tetractinellida 
is sometimes restricted). Spicules four-rayed and not joined 
into a rigid network. The simplest spicule has the form 
of a caltrop (Fig. 11 a, c). In others one ray is elongated, 
forming a shaft from which the other rays project as three 
prongs ; this trident shape is called a triaene and is subject 
to much further modification (Fig. 11 f-k). With these 
D 2 
Gallery X. 
Table-cases 
13, 12, 11. 
Wall-cases 
7, 8a. 
Wall-case 
7b. 
