ASTROSCLERA WILLEYAXA, THE TYPE OF A NEW FAMILY OF SPONGES. 479 



might be present, but that they result from slow molecular changes set up in the 

 trabeculae in continuity with the parts already changed. 



The whole question of the cause of the change and the way in which it has 

 operated is, of course, very obscure, but I have had the opportunity of seeing what 

 appears to be an instance of a somewhat similar process in a specimen of fossil wood 

 in the possession of my friend Mr Seward, and ligured by him in his work on Fossil 

 Plants (Fig. 14, B, p. 81). The siliceous matrix of the specimen has at certain points 

 taken on a definite structure, showing concentric alternating rings of light and dark. 

 In longitudinal sections the structure of the woody fibre is obscurely seen in the regions 

 outside the systems of concentric lines, but within them, precisely where the change in 

 the matrix has occurred, the structure is preserved in exquisite perfection, so that the 

 minute transverse striations on the walls of the vessels are clearly seen. 



There seems therefore to be good reason for believing that in the case of the 

 St Cassian sponges the spherulitic structure is the result of recrystallization and 

 masks a primary spicular structure. The possibility must however be borne in mind 

 of the occurrence of Astrosclera or allied forms in a fossil state. If the fossils retained 

 theii- original structure they would differ from the St Cassian sponges which I have 

 seen, in the absence of traces of a spicular skeleton, and also, I believe, in the strictly 

 radial character of the fibres comj^osing their skeletal elements, and in the uniformity 

 and completeness of the radiating systems thus formed. 



From one point of view the resemblance between the radiate structure of the 

 skeletal elements of Astrosclera, and that of the spherulitic systems of the fossils 

 is interesting ; namely that it is an instance of the formation in living protoplasm 

 of a skeletal structure composed of carbonate of lime in a form almost identical 

 with that which this mineral may take in crystallizing outside a living body, and 

 thus in inorganic surroundings. 



The Family Astroscleridae. 



In view of the isolated position which Astrosclera holds among sponges it must 

 be regarded as the type of at least a separate family. It appears indeed that a 

 larger division will have to be established for its reception, but until specimens are 

 available in which the minute characters of the soft parts can be detennined with 

 certainty, the establishment of such a division may be deferred. 



I propose to call the new Family Astroscleridae. Its characters are, at present, 

 those of the species which constitutes it, namely : 



Sponges which are nearly cylindrical, or expand from a narrow base into a wide 

 disc, — differences in shape which may represent stages of growth. The upper surface 

 is convex. 



The sides and the under surface of the disc, when this is present, are imperforate 

 and more or less conspicuousU' marked by annular wrinkles. 



The upper surface is pitted by closely set pores, the openings of the skeletal canal 

 system. The pores may be irregularly distributed, or larger pores may be arranged 

 in one or more systems of radiating lines. 



64—2 



