130 BRITISH PALEOZOIC SPONGES. 



Hemispherical or ellipsoidal Sponges, with broad, markedly concave bases, 

 and round or somewhat depressed summits. They vary from 50 to 100 mm. in 

 width ; on account of the compressed condition of all the specimens their height 

 has not been ascertained. 



The rhomboidal spicular plates in the basal and zonal regions of the Sponge 

 vary from 3 '5 to 5 mm. in extreme width, whilst near the summit aperture they 

 are not more than 1 mm. wide. The summit aperture is about 3 mm. in width. 



This species is distinguished from Ischadites Koenigii, Murch., by its larger 

 proportions, the concavity of the base, and the somewhat larger dimensions of the 

 spicular summit-plates. Only casts of this species are known, and these 

 generally exhibit the concave basal portion ; but in specimens from the Grindrod 

 Collection, now in the Natural History Museum at Oxford, the upper surface is 

 exposed to view ; and in one instance the casts of the spicular plates are preserved, 

 as shown in fig. 2 a. 



As in the case of I. Koenigii, this species likewise appears to have been 

 gregarious, several individuals occurring on the same slab of rock in close 

 proximity to each other. 



In Bigsby's ' Thesaurus Siluricus ' Ischadites Grindrodi, Salter, MS. is included ; 

 this name may possibly refer to the present species, but as neither specific 

 description nor figure of it has ever been published, and there are no means of 

 definitely ascertaining the type form, I have judged it best to adopt a distinct 

 name for the present species in order to avoid ambiguity. 



Distribution. — Silurian : Wenlock shale, Malvern. Lower Ludlow ; Ledbury. 

 Also in the lowest beds of the Silurian at Petesvik and Hablingbo, Isle of Gothland 

 (Lindstrom). 



Genus. — Amphispongia, Salter. 



1861. Memoirs Geological Survey Great Britain, Sheet 32 Scotland, p. 135. 



Sponges elongated-oval or elliptical in outline, greatly compressed, free, with 

 rounded bases and summits. No traces of a canal-system. The basal portion 

 consists of robust, relatively large, conical spicules, disposed side by side nearly 

 in contact with each other, so that their pointed ends converge to the central axis of 

 the Sponge, whilst their distal rounded summits form the basal surface. The 

 upper portion of the Sponge consists of slender four- and five-rayed spicules with 

 the rays at right angles to each other. These apparently are modified hexactinellid 

 spicules. The spicules are regularly arranged so that their rays lap over and 



