M. K. A. Zittel on Fossil HexactineUida. 269 



mid skeleton of these sponges. The section treating of the 

 siliceous skeleton possesses especial interest. In this Marshall 

 demonstrates that all HexactineUida consist of spicules which 

 in their form follow the axial system of an octahedron. The 

 multifariously complicated or reduced siliceous structures may- 

 be derived from the cleavage or abortion of individual rays or 

 entire axes. 



In the simplest forms, the rod spicules, the cruciform ar- 

 rangement of the three rectangularly crossing axes very fre- 

 quently still shows itself in the axial canals. These axial 

 canals are therefore of particular importance in the Hexacti- 

 neUida. Marshall describes in detail the characters which the 

 axial canals and their cylinders, where such are present, display 

 in the different forms. In general the axial canals are con- 

 siderably wider in the HexactineUida with coherent frame- 

 works than in those with isolated spicules. 



Marshall has further paid particular attention to the union 

 of the siliceous parts in the sponge-body. This is effected, 

 according to Marshall, in three ways : 1, the spicules are 

 united only by sarcode and remain isolated; 2, they are 

 soldered together ; or, 3, they groic together. 



In the " soldering " [Verschmehimg) the spicules are only 

 superficially cemented together by layers of siliceous substance, 

 the axial canals remaining isolated and never flowing toge- 

 ther. In Euplectella aspergiUimi the cementation takes place 

 by laminar new formation of silica, which establishes bridge- 

 like unions between two neighbouring spicules. In Farrea, 

 Aj)hrocanistes, and Fureie, two approximated axial canals are 

 generally surrounded by a common siHceous cylinder. Carter 

 had already called attention (in opposition to Bowerbank) to 

 this peculiarity ; but while the English spongologist denies 

 the occurrence of connected axial canals in neighbouring 

 spicules in the HexactineUida, Marshall thinks he has observed 

 a connected canal-system in the united sexradiates in a single 

 form (Sderothamnus). This phenomenon is described as 

 "growing together" ( Verioachsung) , and a special phylogcnetic 

 significance is ascribed to it. 



In the special part the detailed description of a new Eujjlec- 

 tella {E. Oweni) with free spicules is of the most interest, 

 because here Marshall most convincingly shows that the 

 cementation of all the spicules or of individual spicules in E. 

 as2)ergilluvi and E. cucumer possesses only a quite secondary 

 significance, and that in the three forpis the closest agreement 

 prevails in all essential characters (in the external form and 

 the form and arrangement of the skeleton- and flesh-spicules), 

 so that their generic separation is quite inadmissible. 



