OPHETSTOMA.— CINCLIDERMA. 127 



possession of spicules with lantern nodes, between this form and the fragment of a 

 sponge which T. Smith named V. cavatus. 



Distribution. Green Sand : Cambridge. According to Prof. Seeley it also occurs 

 in the Chalk at Hunstanton, Norfolk. 



Genus PLACOTREMxi, Hinde, gen. nov. 



Sponges forming flattened expansions with rounded margins. The smooth upper 

 surface is perforated with numerous apertures. The dermal layer is formed by large 

 cross-shaped spicules, disposed over each other without any regularity ; the interspaces 

 between their arms are occupied by a very minute spicular mesh. The interior 

 skeleton consists of a spicular mesh arranged so as to form delicate anastomosing 

 laminae. The nodes of the spicules are solid. 



This genus differs from Porospomjia, D'Orb., and Ophrystoma, Zitt., in the dispo- 

 sition of the interior mesh. From this latter genus also, which in general form it 

 closely resembles, it is further distinguished by the different character of the dermal 

 layer and the compact spicular nodes of the interior skeleton. It appears to be allied 

 to Placochlcenia, Pomel ; but as the characters of the spicular mesh of this genus are 

 not given by the author, it is not possible to compare this structure in the two genera ; 

 the dermal layer also differs from that of Placochloenia. 



Placoteema cretaceum, Hinde, n. sp. (Plate XXVII. figs. 4, 4a, ib, 4c.) 



This sponge appears to have grown in flattened masses of an elliptical or ovate 

 outline ; the largest specimen is 60 mm. in length by 50 in width. The wall varies 

 from 7 to 9-5 mm. in thickness. The apertures of the upper surface are ovate, with 

 depressed margins, and disposed either in diagonal rows or irregularly. They are 

 from 2'5 to 4 mm. in width. 



The large cross-shaped spicules of the dermal layer are of various sizes ; the ray 

 of a fairly large specimen measures 1-5 mm. Some, if not all, of these surface- 

 spicules appear to have had a fifth ray, which penetrated the dermal layer at right 

 angles to the surface. The wall-plaits of the interior are -75 mm. in width ; the 

 spicules of the mesh are relatively small, and the distance from node to node is 

 •25 mm. 



The specimens are preserved in chalk ; the spicules have been dissolved, and only 

 empty casts or infiltrations of iron peroxide remain. 



Distribution. Upper Chalk: Kent and South of England {coll. Mantell and 

 Toulm. Smith). 



Genus CINCLIDERMA, Hinde, gen. nov. 



Sponges inverted conical or funnel-shaped. The exterior surface consists of a 

 very smooth dermal layer formed by large cross-shaped spicules, disposed in such a 



