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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 



