124 BRITISH PALEOZOIC SPONGES. 



also of linear spicules, which are grouped together, so that the spicular rays 

 which extend the length of Sponge form, by their apposition and overlapping 

 each other, continuous spicular fascicules, whilst the transverse rays extend singly 

 on either side of the vertical rows, and overlap those from the adjoining rows, so 

 as to produce an irregular framework. Smaller spicules are irregularly interspersed 

 in the intervals between the larger. 



This genus is closely related to Protospongia, Salter, and Dictyophyton, Hall ; 

 but the arrangement of its spicular mesh is far less regular than in either of these 

 genera, and there are no definite quadrate areas. From Protospongia it is further 

 distinguished by the fascicular grouping of the spicular rays in a vertical direc- 

 tion. 



9. Plectoderma scitdlum, Hinde. Plate III, figs. 1, 1 a, 1 b. 



1883. Plectodebma scitulum, Hinde. Cat. Foss. Spong., p. 132, pi. xxxi, figs. 1 



la, lb. 



The fragment of the Sponge- wall which has been preserved is about 60 mm. in 

 width and 45 mm. in height ; it probably formed part of a cup- or funnel-shaped 

 sponge. The vertical fascicles are about 7*5 mm. apart from each other ; there 

 are from five to ten spicular rays in close juxtaposition in each fascicle, but from 

 their present condition it cannot be determined whether they were laterally 

 cemented together or not. The spicules vary greatly in size ; the rays of the 

 slender forms are not more than '09 mm. in thickness, whilst in some of the larger 

 forms the rays are '35 mm. in thickness, and 6*5 mm, in length. The rays are 

 straight, curved, or occasionally wavy, they usually taper very gradually from the 

 central node to their extremities. Traces of smaller spicules can be occasionally 

 seen in the interspaces between the larger, and it is probable that they formed 

 part of a continuous spicular membrane extendiDg between the framework formed 

 by the larger spicules. 



The type-specimen, the only one hitherto discovered, is preserved on the 

 surface of a soft, micaceous, shaly rock. The spicules are now mostly represented 

 by empty casts, but in some cases fragments of the original siliceous spicules, still 

 retaining their axial canals, are preserved. The wall at first sight appears to 

 consist of rod-like spicules, crossing each other at right angles, but where the 

 spicules are less thickly grouped together the four rays of a spicule springing from 

 a centre can be distinctly seen, and at the common centre there is often a small 

 circular aperture, indicating a fifth ray extending inwards at a right angle to the 



