496 THE VOYAGE OF H.M.S. CHALLENGEr.. 



beneath the sea-level, or in still unexplored regions. Under these circumstances it is not 

 yet possible to construct a genealogical tree for the various genera, so we must dismiss to 

 a pre-Silurian time, where the light of experience no longer guides us, all the hypotheses 

 which would derive the Hexactinellida from the Tetractinellida or Monactincllida." 



An explanation of the sudden appearance of the fossil Lithistida3 and Hexactinellida 

 may be reasonably sought in the mode of life exliibited by their modern representatives. 

 Both groups are especially inhabitants of the deep-sea, and only in the former deep-sea 

 deposits can one hope to find their fossil remains in any abundance. 



If therefore we have to refer the origin of the Hexactinellida to a pre-Silurian period 

 of which no certain representatives are preserved, we cannot obviously expect from 

 palaeontology any conclusion as to the manner of their origin or the nature of the primitive 

 forms. It is, however, of importance, that even in the Silurian period both Lyssacine and 

 Dictyonine forms appear to be present. In the later mesozoic formations which are so 

 rich in Dictyonina (especially the Jurassic and Cretaceous), hardly a hint of Lyssacina is 

 to be found, while now they form among living forms a large majority — a fact which 

 appears to find its explanation in the improbability of the preservation of foi-ms with a 

 loose spicular framework which would fall to pieces with the destruction of the soft parts. 

 The following considerations may, however, appear not unworthy of close examination. 



From the fact that the modern Hexactinellids all live at a considerable depth, always 

 below 95 fathoms, Zittel has already drawn the conclusion that they were probably from 

 the first deep-sea forms. 



From the bathymetrical statistics of the Challenger Hexactinellida, it is seen that the 

 Hexactinellids now living in the greater oceanic depths are almost exclusively Lyssacina, 

 and that the modern Dictyonina are, with the exception of one very simple genus, all 

 confined to the relatively lesser depths, between 100 and 1500 fathoms. The conclusion, 

 . therefore, seems warranted, that in ancient times also the Lyssacina predominantly occurred 

 in the greater depths, while the more differentiated Dictyonina inhabited, as they now do, 

 relatively shallower waters, at no very great distance from the coasts. Now if one 

 may assume that the deej^est regions of the great oceans have remained permanently 

 covered with water since the Palteozoic period, while only the shallower regions near the 

 continents were here and there raised above water, and thus became now accessible to the 

 hammer of the palseontologist, we can understand why we find in certain Jurassic and 

 Cretaceous deposits so many and highly differentiated Dictyonina, but only slight hints 

 of Lyssacina, even in circumstances which would not exclude their preservation, or at least 

 that of their characteristic spicules. 



Hitherto I have restricted myself to the Hexactinellida themselves ; I shall now consider 

 their relation to the other groups, beginning with the calcareous sponges. 



It may be assumed as a generally acknowledged fact that tlie calcareous sponges are 



