90 Mr. J. S. Bowerbank on Moss Agates, fyc* 



dony, the effect is different ; the radiating crystals of the chal- 

 cedony are then frequently found to have their terminations 

 surrounded by a mass of molecules of the decomposed 

 spongeous matter which has been driven before them during 

 the process of crystallization, in the same manner that the de- 

 composed cellular structure of fossil wood is frequently ob- 

 served to be driven before the radiating crystals either of silex 

 or carbonate of lime, whichever may form the fossilizing me- 

 dium ; and it not unfrequently happens, that both modes of 

 arrangement of the siliceous matter may be observed in the 

 same specimen, the radiating or chalcedonic arrangement of 

 crystals being often based upon the agatized portion of the 

 specimen. 



In conclusion, I may be allowed to observe, that there are 

 circumstances attending the elucidation of the subjects treated 

 of in this paper, which envelopes them in a greater degree of 

 difficulty than that which attends the investigation of other 

 organic remains, inasmuch as the structure of recent sponges 

 has been very little studied by modern naturalists, and then, 

 excepting in very few instances, only in such a state and 

 manner, as to throw, comparatively speaking, but very little 

 light on their structure, either in the recent or fossil condi- 

 tion. The aspect of a spongeous body, when viewed without 

 the assistance of a high magnifying power, is so widely dif- 

 ferent from its appearance beneath the microscope, as to ren- 

 der it highly probable that it would never be identified in the 

 fossil state, unless the eye of the observer had been previously 

 well practised in the investigation of the structure of the re- 

 cent sponges, as well as of the fossil ones ; and even then it 

 must be remembered that we are viewing but the skeleton of 

 the sponge. In the recent keratose species, the horny fibres, 

 when alive, are surrounded by a mucous coat, and imbedded 

 in fleshy matter, very little of which can be expected to remain 

 in situ in the fossils ; and we can only hope to find but ob- 

 scure indications of its remains in the form of a turbid semi- 

 decomposed mass, in which the more durable parts of the ani- 

 mal are imbedded, preserving, in some instances, their pris- 

 tine form and beauty ; more frequently in such a disorganized 

 and confused state, as to surround their identification with 

 many doubts and difficulties, and to require much patient in- 

 vestigation, and an acquaintance with their recent types, both 

 in a state of perfect preservation and of nearly entire decom- 

 position. 



That the remains of sponges thus found in such abundance 

 should almost in every case prove to be those of the keratose 

 tribe, is what we might naturally expect to be the case ; as in 



