PLATE XXXI. 



Page 

 Fig. 1. Plectoderma scitulum, Hinde. A fragmentary specimen, natural size, from Silurian 



strata of Upper Ludlow age, in the Pentland Hills, near Edinburgh. For the 

 most part only empty moulds of the spicules are preserved, but here and there 

 fragments of siliceous spicules yet remain. The specimen figured belongs to 

 the collection of the Geological Survey of Scotland. It was discovered by 



Mr. James Bennie 132 



Fig. la. The same. A portion of fig. 1, enlarged five times, showing the arrangement of 

 the spicules. Owing to the uneven surface of the matrix the vertical axes of 

 the spicules appear as if frequently broken off. 

 Fig. 1 b. The same. A single spicule, enlarged five times, showing the complete vertical 

 axis and one complete ray, and the commencement of the other ray of the 

 horizontal axis. 

 Fig. 2. Diplodictyon Bayfieldi, Hinde. Natural size. From the Upper Chalk at Norwich. 

 The specimen is preserved in chalk, and only casts of the spicular structure are 



preserved 145 



Fig. 2 a. The same. Showing the compressed sides of fig. 2, with the large lateral apertures. 

 Fig. 3. SclerokaUa Cunningtoni, Hinde. An imperfect specimen, natural size, showing 

 the interior, and sections of the walls. From the Upper Green Sand at War- 

 minster, Wiltshire. The specimen belongs to the Jermyn-Street Museum . . 146 

 Fig. 3 a. The same. A portion of the dermal layer of the interior of the cup. Enlarged 



thirty times. 

 Fig. 3 b. The same. Part of the spicular structure of the outer surface of the cup. Enlarged 



thirty times. 

 Fig. 3 c. The same. A portion of the spicular mesh of the interior, immediately beneath 

 the dermal layer, showing its regular arrangement and the octahedral character 

 of the nodes. Enlarged thirty times. 

 Fig. 4. Coeloptychium agaricoides, Goldf. Showing the upper surface of the body of a 

 small imperfect specimen from the Upper Chalk of the South of England. The 

 spicular structure is now only indicated by red markings of iron peroxide in the 



chalky matrix. Natural size 146 



Fig. 4 a. The same. A vertical section of fig. 4, natural size, showing in section the folds 



of the walls in the interior of the sponge. 

 Fig. 5. Astraospotigia patina, Roemcr. Detached spicules, enlarged twenty-eight times, 

 from decayed Silurian shale of Wenlock age, near Visby, in the Island of 

 Gotland. The spicules have been changed to calcite. A small quantity of the 

 shaly matrix is usually attached to the central portion of the spicule . . . .149 

 Fig. 6. Stauraciinella cretacea, Hinde. An imperfect specimen from the Upper Chalk. 

 Natural size. Traces of free hexactinellid spicules, now in the condition of iron 



peroxide, are shown on the outer surface 149 



Fig. 7. Hyalostelia fusiformis, Hinde. Free hexactinellid spicules with inflated centres. 



From the Upper Chalk at Horstead, Norfolk. Enlarged twenty-eight times . 151 



