PLINTHOSELLA. 85 



Relying on the similarity in the spicular structure of the interior mesh, and on 

 the general resemblance of the disks of the dermal layer of these cup-shaped, clavate, 

 and convolute forms to the same structures in the spherical P. squamosa, I shall 

 place them under this genus, and propose to extend the definition given by Zittel so 

 as to include sponges of these forms. The new forms, like the type species, have 

 been derived from the Upper Chalk. 



Plinthosella squamosa, Zittel. (Plate XX. fig. 2.) 



1878, Plinthosella squamosa, Zittel, Studien, II Ab. p. 89, t. 2. f. 10 and t. 10. f. 5 ; also 



Hauclbucli, p. 1G7, f. 80. 

 1880. Plinthosella squamosa, Hiude, Foss. Sponge-Spicules, p. 56, t. 4. f. 35—46. 



Small spherical bodies, from 6 mm. to 23 mm. in diameter, apparently destitute of 

 any point of attachment. 



In some examples there appear to have been a few canals immediately beneath 

 the dermal layer, but the circulation in the interior of the sponge seems to have been 

 carried on in the interspaces of the mesh. 



The spicular structure of the interior is composed of relatively large, irregularly 

 four-armed spicules with tuberculated surfaces and minute twig-like extremities, by 

 which they are interlocked together. There is a dermal layer of small, irregularly 

 shaped disks or laminse which are arranged so as to overlap each other. 



The examples of this sponge occur in this country as small globular bodies, from 

 the size of peas to that of marbles, inclosed in the interior of Chalk flints. In all 

 the specimens which I have seen the dermal layer is absent, and only the empty 

 cavities of the spicules of the interior labyrinthine mesh remain. When closely 

 examined with a strong lens, some of the moulds, still retaining the impressions of 

 the tubercles on the surface of the spicules, can be seen. In the Horstead Chalk I 

 discovered detached spicules both of the interior and of the dermal layer of this 

 species. 



Bistrihution. Upper Chalk : South of England ; Wiltshire [Mr. H. J. Carter) ; 

 Horstead, Norfolk (detached spicules) ; Ahlten, Hanover (Zittel's coll.). 



Plinthosella compacta, Hinde, n. sp. (Plate XX. figs. 3, 3 a.) 



Body of sponge cup-shaped, supported on a cylindrical stem. The interior is 

 nearly filled with the spicular mesh, so that there is only a shallow depression at the 

 summit of the cup. The only specimen is 50 mm. in height, and 72 mm. in thickness 

 near the summit. 



The rounded margins of the cup are seamed with sinuous transverse canals ; and 

 the outer surface is covered with apertures, 1 mm. wide, of canals which extend into 

 the sponge-wall. A few canals also extend into the stem. 



