48 SILICEOUS SPON&ES. 



divides and subdivides dichotomously ; the branches are cylindrical or slightly 

 compressed, approxiinately of the same thickness throughout, and from 14 mm. 

 to 19 mm. in diameter in different specimens. The summits of the branches are 

 rounded. An average specimen is 250 mm. in height, with an expansion of 190 mm. 

 across the summit. 



The branches are traversed longitudinally by a series of simple cylindrical tubes, 

 2 mm. each in width. No regular canal-openings appear to be present on the lateral 

 surfaces of the branches; and the circulation seems to have been carried on between 

 the irregular interspaces of the mesh, which average "5 mm. in width. 



The arms of the spicules are '09 mm. in thickness ; they are very compactly inter- 

 woven together so as to form a very close mesh. No dermal spicules have been 

 detected in this species. 



This species may be distinguished from D. ramosum and I). Ecemeri by the great 

 regularity in its mode of growth and the close character of the spicular mesh. It 

 appears to be abundant. 



Distribution. Upper Green Sand : Warminster, Wiltshire. 



DoKTDERMA RAiiosuM, Mantell, sp. (Plate VIII. figs. 2, 2 a.) 



1822. Spongia ramosa, Mantell, Fossils of the South Downs, p. 162, t. 15. f. 11. 

 1808. A ramose Alcyonite, Parkinson, Organic Remains, vol. ii. t. 7. f. 6. 

 1878. Doryderma ramosa, Zitt. Stud. II Ab. p. 68. 



Sponge branching, growing from an upright cylindrical stem, which divides and 

 subdivides occasionally, giving off short stumpy branches, from 12 to 18 mm. in 

 diameter. The extremities of the branches are rounded. The vertical canals are 

 about 1'5 mm. in width; they can hardly be distinguished, owing to the imperfect 

 preservation of the interior structure of the sponge. The interspaces of the mesh on 

 the surface are "6 mm. wide. The spicular arms are 'ISTS mm. in thickness. No 

 surface-spicules have been preserved. 



The type of Mantell's species is in the Museum collection. Its structure has been 

 so completely altered by fossilization, that it was only after repeated scrutiny that I 

 discovered a single spicule and was thus enabled to determine its true characters. 

 The branching sponges which, from their similarity of form, I refer to this species 

 are not uncommon in the interior of flints in the Chalk of Wiltshire. The surface- 

 structure is usually well preserved ; but the interior characters are mostly destroyed. 

 This species is less regular in its mode of growth, and the spicules and mesh-inter- 

 spaces are larger than in D. dickotomum, Benett, whilst the stems are usually smaller, 

 and the mesh-interspaces also, than those of B. Ecemeri, Hinde. 



Distribution. Upper Chalk: near Brighton ; Oare, Wiltshire. 



