PRINCIPAL GROUPS OF THE SPONGES. 173 



" summit-plates,' 5 which are nearly in contact, and are arranged in 

 obliquely curved intersecting rows, giving the external surface of the 

 fossil very much the appearance of the engine-turned case of a watch. 

 Internal to the summit-plate, each radial spicule gives off four transverse 

 or horizontal rays, as in the genus Receptaculites ; but the inner ends of 

 the radial spicules simply terminate in pointed extremities, and there is, 

 therefore, no internal plated membrane such as characterises the latter 

 genus. The species of Iscliadites are found in the Ordovician rocks, 

 and, more abundantly, in Silurian deposits, but they are not known to 

 have survived into the Devonian period. Nearly allied to Iscliadites is 

 the genus Acanthoc1io)iia, which is also of Silurian age. Lastly, the 

 genus Sphcerospongia includes pyriform or cup-shaped organisms, the 

 outer integument of which consists of regularly arranged hexagonal 

 calcareous plates, which represent the expanded outer ends (or " summit- 

 plates ") of a series of Hexactinellid spicules. Immediately below, or 

 internal to, the summit-plate, each spicule gives off four transverse or 

 horizontal rays, but the actual stem of the spicule, or, in other words, its 

 radial ray, is either aborted or is represented only by a short knob-like 

 projection. The species of Sph&rospongia are confined, so far as known, 

 to the Devonian rocks. 



In spite of their abnormal form and structure, as compared with recent 

 types, there seems to be no reason for doubting that Dr Hinde is correct 

 in his view that the genera Receptaculites, Iscliadites, Acanthochonia, and 

 Sphcerospongia constitute a peculiar group of Lyssakine Hexactinellid 

 Sponges, all the members of which are Palaeozoic. 



In rocks of Silurian age, the Hexactinellid Sponges are represented 

 not only by the Anchoring-Sponges belonging to the genus Hyalo- 

 stelia and by the abnormal genera of the Receptaculitidaz just noticed, 

 but also by the curious types known as Dictyophyton and Plectoderma. 

 The genus Dictyophyton comprises cylindrical or cup-shaped sponges, 

 apparently not attached to foreign bodies, in which the wall consists 

 of a connected spicular framework disposed so as to form a series of 

 quadrangular areas. The precise structure of the spicules and their 

 mode of union are points as yet imperfectly known. The species of 

 Dictyophyton are not only found in the Silurian rocks, but likewise 

 in Devonian strata, and species occur also in the Carboniferous 

 deposits. The Silurian genus Plectoderma is related to Dictyophy- 

 ton, but the spicular network is much less regular. 



In the Devonian deposits, the groups of Hexactinellids represented 

 by Sphcsrospongia and Dictyophyton still persist, but our knowledge 

 of other types is very imperfect. In the Carboniferous period, again, 

 the Hexactinellids are chiefly represented by the genus Hyalostelia, 

 the anchoring fibres of which are not very uncommon in the lower 

 beds of the system ; but other genera of the Lyssakina (such as 

 Holasterella) are likewise represented. 



In the Permian and Triassic systems, so far as our certain know- 

 ledge goes, Hexactinellid Sponges may be said to be still unknown ; 

 though the imperfectly examined Bothroconis of King, from the Per- 

 mian of the North of England, may possibly belong here. 



