Dr. G. J. Hinde on Fossil Calcispongic. 187 
to determine its original composition, as in the case of the 
siliceous sponges just mentioned. In all existing Calcisponges 
the spicules forming the skeleton are imbedded in the exo- 
derm or syncytium of the sponge, and never form, as in the 
ease of the Pharetrones, anastomosing solid fibres. Thus 
loosely imbedded in the sarcodic material of the syncytium, 
it would be impossible that they should be able to retain their 
arrangement after the death of the animal and the consequent 
destruction and decay of the syncytium. This perhaps led 
Hiackel to state in his magnificent monograph that up to the 
present (1872) not a single fossil calcisponge is known (vol. i. 
p- 241). He further adds, “One might expect to find the 
microscopic calcareous spicules of these sponges as fossils in 
different strata; and probably with more complete micro- 
geological investigations these will be abundantly found ; but 
up to the present none have ever been described and figured.” 
If, however, it can be shown that the skeleton of the fossil 
Pharetrones, though disposed in the form of solid anasto- 
mosing fibres, is yet built up of calcareous spicules which 
show the closest agreement in form with those of existing cal- 
cisponges, the objection raised against their alliance with the 
existent examples of the order would be deprived of most of 
its importance; for it is on the character of the individual 
spicules of the skeleton rather than on their arrangement that 
the classification of both siliceous and calcareous sponges, 
recent as well as fossil, depends. ‘The following observations 
of Hickel with respect to the spicules of the existing Calci- 
spongie will equally apply to the fossil examples of the order. 
He says that, notwithstanding there is much less variety in 
their form, arrangement, and combination than in those of 
siliceous sponges, yet they are of the highest morphological 
value, and afford the only sure foundation for a natural classi- 
fication and a genealogical system of the calcareous sponges. 
In studying the characters of the fossil Calcisponges we 
encounter a difficulty arising from the frequent obliteration 
of all traces of spicules in the fibres through the changes 
which have taken place in the process of fossilization. In 
many instances the fibres only show a structure of finely 
radiate calcitic crystals ; in others but faint traces of spicules 
here and there can be detected; and even where the spicular 
structure is fairly preserved the only method available for 
examination is by means of thin microscopic sections, which, 
except in the cases of the simplest forms of spicules, and then 
only when the plane of the section corresponds with that of 
the spicule, do not allow a satisfactory investigation of the 
entire spicule. When only thus partially seen, some of these 
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