86 SILICEOUS SPONGES. 



The spicular structure of the interior closely resembles that of P. squamosa. No 

 dermal layer has been preserved. 



I have only seen one example of this species, which is from the interior of a flint. 

 Notwithstanding the absence of a dermal layer, I have but little doubt of the 

 propriety of placing it in this genus on account of the close resemblance of the 

 interior-mesh spicules to those of the type species. The specimen in question 

 belongs to the Jermyn-Street Museum. 



IHstrihution. Upper Chalk : Wiltshire {Cunnington coll.). 



Plinthosella nodosa, Hinde, n. sp. (Plate XX. figs. 4, 4 a.) 



Sponges simple, rod- or club-shaped, with irregular nodose and spui'-like projections. 

 A fairly large specimen is 100 mm. in height and 80 mm. wide. 



The surface of the sponge, where the dermal layer is not present, is traversed by 

 sinuous canals from -5 to 1 mm. in width. The interior structure is obliterated, so 

 that I am unable to determine if interior canals are j^resent. 



The spicules of the interior mesh resemble those of P. squamosa, but they are 

 somewhat smaller. The surface is covered with a dermal layer of disciform spicules 

 (about '6 mm. in width), very variable in figure, some being deeply lobed like those 

 of Pagadinia, whilst others are circular disks with merely jagged edges. These 

 spicules overlap each other in such a manner as to produce a compact, smooth 

 surface-layer. 



In some respects this species exhibits characters similar to those oi Pagadinia ; and 

 without an examination of the spicular structure, it might readily be confounded with 

 P. clavata. As the spicules of the interior skeleton are of the same type as Plintho- 

 sella squamosa, it appears to me to belong rather to this genus than to Pagadinia. 

 The variable character of the spicules of the dermal layer is well exhibited in this 

 species, for in close proximity to each other on the surface there are nearly plain 

 disks and deeply-lobed branching spicules. 



Distrilution. Upper Chalk: "Wiltshire {Cunnington coll.). 



Plinthosella convoluta, Hinde, n. sp. 



Sponge growing as an irregularly convolute plate from 5 to 5"5 mm. in thickness. 

 The only specimen, which is incomplete, is 110 mm. in height and 82 mm. in lateral 

 extension. 



It is doubtful whether canals are present in the sponge-wall. The skeletal mesh 

 is composed of spicules closely resembling in size and form those of Plinthosella 

 compacta. The dermal layer is absent; but in one small portion of the outer 

 surface there are a few minute scaly spicules which may represent part of the former 

 external layer. 



This species very closely resembles, in its mode of growth, Phymaplectia irregularis ; 



