— 177 — 
In endeavouring to classify these, for the most part, iso- 
lated spicules, I have compared them with the valuable collection 
both of recent and fossil sponges contained in the Palaeontological 
Museum at Munich, (which was freely placed at my disposal 
for this purpose by Professor Zittel) and with the figures and 
descriptions of sponge spicules given by Carter, Zittel, Oscar 
Schmidt, Sollas, Bowerbank and other authors who have de- 
voted attention to the microscopic structure of the sponge 
skeleton. The multitude of works and descriptions of both 
earlier and later writers on fossil sponges, which are mainly 
based on the outer form of the organism, leaving undescribed 
the character of the constituent spicules, were of no value to 
me whatever, since my material only consisted of the small 
bodies by whose combination the sponge itself is formed. 
It has been only within the last few years that the fundamen- 
tal structure of sponges has been studied and the true cha- 
racter of the spicular skeleton recognized and made the basis 
of classification, and though very much remains to be done 
in this field of investigation, the knowledge which has been 
already obtained of the spicular structure of fossil sponges, 
mainly through the researches of Zittel, Carter, and Sollas, 
has made it possible in many instances, to determine the genus 
of sponges to which these isolated and scattered spicules be- 
long. There are however several difficulties, in connection 
with the determination of the relationship of these detached 
spicules, which only allow an approximate conclusion to be 
made as to their affinities. One cause of this uncertainty 
arises. from the fact that in the skeletons of both fossil and 
recent sponges, very many spicules get intermingled with the 
sponge, which have only found their way in the structure by 
mechanical means and are not integral portions of the organ- 
ism, though they have frequently been mistaken for such, 
and have given rise to the belief that in the same sponge 
there could exist the most varied collection of spicules. 
Another difficulty is owing to the fact of the same form of 
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