PLATE XXVII. 



Page 



Fig. I. Sesirocladiafurcatus, Rinie. Natural size. From the Grey Chalk, near Dover . 117 



Fig. 1 a. The same. A transverse section of one of the branches, showing the thickness of 

 the walls. Natural size. 



Fig. 1 b. The same. Portion of the interior of the wall, showing traces of the spicular mesh 

 with the octahedi'al nodes. The spicules have been replaced by calcite, and are 

 partially obliterated. Enlarged thirty times. 



Fig. 2. Pohjblastidium racemosum, T. Smith, sp. A specimen preserved in flint, from the 

 Upper Chalk of Kent, showing on the outer surface transverse sections of the 

 bud -like tubular projections. Natural size 119 



Fig. 2 a. The same. Transverse section of one of the tubes, enlarged four times, showing 

 the foldings of the sponge- wall and traces of the spicular mesh, which is par- 

 tially preserved in iron peroxide. 



Fig. 3. Cephalites bullattts, T. Smith. A specimen partially imbedded in Chalk, showing 

 the characters of the outer surface, and, where this has been removed, traces of 

 the interior folds. The structure is now preserved in iron peroxide. Natural 

 size. From the Upper Chalk of the South of England 122 



Fig. 3 a. The same. The summit of the same specimen, showing the cloacal aperture and 

 one or two of the canal-apertures which open into it, and also sections of the 

 waU-plaits in the interior of the sponge. The dermal layer which covered the 

 summit originally is not preserved. Natural size. 



Fi"-. 3 b. The same. Fragments of the spicular mesh forming the waU-plaits. The spicules 

 have been replaced by iron peroxide, and their outlines are very imperfectly 

 preserved. Enlarged thirty times. 



Fig. 4. Placotrema cretaceum, Hinde. A specimen partially imbedded in chalk, showing 

 the upper surface with the apertures in the dermal layer. Natural size. From 

 the Upper Chalk of Kent 127 



Fig. 4 a. The same. Portion of a transverse section of another specimen, natural size, 

 showing the reticulate disposition of the interior wall-plaits, which are shown 

 by markings in iron pei-oxide. 



Fig. 4 b. The same. Portion of the dermal layer of the upper surface of the sponge, enlarged 

 fifteen times, showing the spicular structure. The spicules have been completely 

 dissolved, and are only represented by hollow moulds in the chalky matrix. 



Fig. 4 c. The same. Portion of the spicular mesh of the interior wall-plaits, enlarged 

 thirty times. Only the empty moulds of the spicules are preserved. Drawn 

 from a fractured surface. 



