SKELETAL STRUCTURES. 83 
In the Dictyonine division of hexatinellids, the overlapping rays of adjoining 
spicules are enclosed and united with each other in a common, regular, even 
coating of silica, so as to form a continuous fibrous meshwork, the faces of which 
may be squares (Fig. 6,/) or polygons. The individuality of the component spicules 
in this continuous meshwork cannot be recognised on the exterior, but it is shown 
by the internal canals of the rays, which can be seen to extend from each of the 
nodes of the mesh and to overlap those proceeding from adjoining nodes. In 
some instances the internal canals appear to be continuous from node to node, as 
in Sestrodictyon (Fig. 6, e), but this probably arises from the breaking down in the 
fossilization of the delicate partitions dividing them. The regular quadrate or 
cubical arrangement of the spicular meshwork is frequently interrupted by the 
irregular interposition of spicules, the rays of which may become fused to the 
nodes or centres of other spicules, or even to the lateral portion of their rays, 
producing a confused meshwork, in which the original hexactinellid form of the 
spicules is largely masked. ‘his irregular structure is more especially developed 
where the spicular structure is minute, and there are numerous canals in the 
sponge-wall, as in Leptophragma, Zitt., and Coscinopora, Goldf. 
The union of the spicules in the dermal layer of hexactinellids in most cases 
differs very considerably from that of the interior meshwork of the skeleton. 
This dermal layer, in some instances, consists of a delicate siliceous membrane 
with circular or polygonal apertures. No individual spicules can be seen on the 
exterior of this membrane, but when examined by transmitted light the membrane 
is seen to consist of a framework of irregularly-scattered four- or six-rayed 
spicules, the canals of which still remain, and between these the delicate membrane 
has been deposited. This kind of dermal layer is shown in Craticularia, Zitt., 
Guettardia, Mich., Plocoscyphia, Reuss, and other genera. It is not merely 
limited to the outer and cloacal surfaces of the Sponge, but frequently lines the 
canals and inter-canals. In another modification, the dermal layer consists of 
large cruciform or five-rayed spicules, irregularly disposed and soldered together 
where the rays touch or cross each other; sometimes also connected by siliceous 
balks or rods or even by a siliceous membrane, as in the genus Cypellia, Pom. 
Sometimes also the skeletal mesh of the Sponge is irregular, whilst the spicules of 
the dermal layer form a very regular connected quadrate meshwork, as in 
Casearia, Quenst. In the existing genus Sclerothamnus, Marshall (= Dendro- 
spongia, Murie), the spicules of the dermal layer are regularly disposed to form a 
quadrate meshwork, but the rays are not cemented together, and remain free, 
similar to those of the dermal layer of Lyssakine Sponges; thus in this genus the 
skeletal mesh is formed by united spicules, and therefore Dictyonine in character, 
whilst the dermal layer is distinctly Lyssakine. In the Cretaceous genus Cincli- 
derma, Hinde, there is a dermal layer of large spicules forming a regularly 
