PLATE XXI. 



Page 

 Mg. I. Phymaplectia irregularis, Rinie. Natural size. Inclosed in a flint from the Upper 



Chalk of Wiltshire 87 



Fig. 1 a. The same. Portion of the surface of the same specimen, enlarged thirty-two times, 

 showing the heads of trifid spicules of the dermal layer. 



Fig. 2. Phymaplectia spinosa, Hinde. Natural size. Inclosed in a flint from the Upper 



Chalk of Wiltshire. The specimen belongs to the Jermyn-Street Museum . . 88 



Fig. 2 a. The same. Portion of the surface of fig. 2 where the dermal layer is absent, 

 showing the spicular structure of the sponge-wall. Enlarged thirty-two times. 



Fig. 2 b. The same. Portion of the surface of fig. 2 where the dermal layer has been 

 preserved, showing some of the heads of its component spicules. Enlarged 

 thirty-two times. 



Fig. .3. Phymaplectia cribrata, Hinde. Natural size. From the interior of a flint from 

 the Upper Chalk at Oare, Wiltshire. The specimen belongs to the Jermyn- 

 Street Museum 88 



Fig. 3 a. The same. Portion of the outer surface of fig. 3, enlarged thirty-two times, showing 

 some of the minute spicular heads and the pores of the dermal layer. 



