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. This 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 Graticularia, 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 Gypellia, 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 Gincli- 

 derma, Hinde, there is a dermal layer of large spicules forming a regularly 



