544 PROF. W.J. SOLLAS ON" SOME FOSSIL SPONGES 



and other soft tissues ; on the other hand, this would not prevent 

 the entrance of mineral solutions, which, penetrating the interior, 

 might deposit in the vacant canals calcite or silica, as the case might 

 be. Subsequently the organic matter would be dissipated by decom- 

 position, and the fine calcareous mud, no longer excluded, would be 

 able to insinuate itself into every space left vacant by the disap- 

 pearance of organic matter, and so to fill up the cavity which would 

 intervene between the thread of calcite or other mineral which had 

 been deposited in a canal and the walls of the surrounding skeleton. 

 Thus the problematical threads of our fossil sponges may be re- 

 garded as representing the original cavities of the canals ; while the 

 consolidated mud or empty space, as the case may be, has taken the 

 place of once existing gelatinous connective tissue. How far this, 

 which is the only explanation I can suggest, is the true one, it is 

 hard to say. One difficulty on the face of it is the rapidity which 

 it seems to require in the rate of deposition of the minerals now 

 forming the axial threads ; but till we know more about the dura- 

 bility of certain organic tissues, and the rapidity with which mineral 

 deposition takes place in shallow warm seas, this objection cannot 

 be regarded as a fatal one. 



The last constituent of the matrix of this sponge which remains 

 to be mentioned is haematite, which occurs dispersed through certain 

 patches of calcite in minute blood-red globules like those described 

 in my paper on the Silurian District of Rhymney (Q. J. G. S. vol. 

 xxxv. p. 504). 



Mastodictyum, gen. nov. 



A branching, leaf-like expansion, bearing on the upper surface 

 numerous mamillary processes, or teat-like individuals, resembling 

 diminutive Emplocce. Each is perforated axially by a cylindrical 

 canal, extending nearly to the base, and opening above in an apical 

 oscule with a sharply defined circular margin. Incurrent ostia are 

 dispersed at about equal distances over the exterior, and the in- 

 current and smaller excurrent canals run through the walls at right 

 angles to the surface, alternating with each other. Skeleton 

 Euretid ; dermal skeleton absent. 



M. Whidboknt, sp. nov. (Plate XX. figs, 7-9.) 



The single specimen on which this species is founded is a fan-like 

 expansion, 3 inches long by 4 inches wide, with mamilke about 

 | inch high rising from it. The walls of the mamillae are about 

 J inch thick, the single osculum to each from ^ to \ inch in diameter ; 

 the incurrent ostia are from -^ to Jg- inch in diameter. 



Loe. Burton Bradstock. 



Hor. Upper part of A. Parlcinsoni-zone. 



Mineral Characters. The description given for the previous species, 

 Emploca ovata, as regards the characters of the skeleton and infilling 

 material, will apply equally well here, except in respect to a few 

 details, such as the presence of silica in JEmploca, which does not 

 occur in this sponge. In this, however, and most of the succeeding 



