172 



PORIFERA. 



into rhomboidal "summit plates" (fig. 61, b), which fit into one 

 another so as to form a mosaic-like outer layer or membrane, the 



plates being disposed in de- 

 cussating lines, Immediately 

 below, or internal to, the sum- 

 mit-plates, the vertical spicules 

 give off four transverse or hori- 

 zontal rays, which only become 

 visible externally when the sur- 

 face-plates are worn away or 

 are viewed from the inside (fig. 

 6 1, b). At their inner ends, 

 where they abut upon the 

 central cavity of the organism, 

 the vertical spicules expand so 

 as to form small horizontal 

 plates (fig. 62, b), which are 

 traversed by horizontal canals, 

 and are so apposed to each other 

 as to give rise to an inner cal- 

 careous membrane. In one 

 species of the genus, if not in 

 all, the plates of this inner 

 membrane are so disposed as to leave cylindrical canals at their 

 angles of junction, through which the water which has filtered be- 

 tween the summit-plates of the outer layer of plates is admitted to 

 the internal cavity of the organism. In most specimens of Recep- 

 taculites the skeleton-spicules are now composed of calcite or per- 

 oxide of iron, but Hinde has adduced evidence to show that the 

 spicules were originally siliceous, and that their present condition is 

 the result of replacement. The species of Receptaculites are widely 

 distributed in the Ordovician and Silurian deposits, and are also 

 well represented in the Devonian rocks, while a species is recorded 

 as occurring in the strata of Carboniferous age. 



The genus Ischadites x comprises fossils which are in many respects 

 closely allied to Receptaculites, but in which the central cavity is roofed 

 over superiorly, instead of being widely open as it is in the latter genus. 

 The form of the organism is, therefore, that represented in the diagram- 

 matic figure of Receptaculites given above (fig. 62), being conical or 

 pyriform, usually with a summit-perforation opening into the central 

 cavity. The wall of the organism is composed of cylindrical, pillar-like 

 spicules, arranged at right angles to the surface, and in most respects 

 similar to the corresponding structures in Receptaculites. The outer 

 ends of these hexactinellid spicules are modified to form rhomboidal 



Fig. 62. — Diagram of the structure of Recep- 

 t acuities, as it would be shown by a vertical sec- 

 tion of a perfect specimen. a, The aperture at the 

 summit ; b, The inner plates of the spicules; c, 

 The summit-plates of the spicules ; n, The usual 

 position of the nucleus ; v, The great internal 

 cavity. The unshaded bands running from the 

 outer to the inner integument represent the spic- 

 ules. (After Billings.) The form of this figure 

 agrees with that of Ischadites, the body in Recep- 

 taculites being of the nature of an open cup or 

 bowl. 



1 The fossils to which the name Tetragonis have been given belong in part to 

 Ischadites, and are in part referable to the Hexactinellid genus Dictyophyton. 



