M. K. A. Zittel on Fossil llexacilaellida. 407 



place as Monakidte in opposition to the Pleionakidje and 

 Pollakidse. 



The second group, that of the Dictyonina, contains the 

 Hexactinellida with regularly amalgamated sexradiates. In 

 normal development the amalgamation is effected by each arm 

 of a spicule applying itself closely to the corresponding arm 

 of a neighbouring sexradiate, when the two rays are soldered 

 togetlier by a common siliceous envelope which is deposited 

 uniformly around them, and they become so completely amal- 

 gamated that their previous independence is indicated only 

 by the presence of two separate axial canals. In this way 

 regular connected latticed frameworks are produced, in which 

 each beam consists of two arms of two different spicules. 

 Frequently, however, irregularities in the arrangement of the 

 openings of the latticework are caused by a sexradiate to a 

 certain extent quitting the ranks, and cementing its arms in 

 an arbitrary fashion to the rest of the framework. If one or 

 two rays of such irregularly placed spicules attach themselves 

 accidentally to the thickened centre of intercrossing of a sex- 

 radiate, more than six arms may issue from such a central 

 point. Careful examination, however, always shows that the 

 supernumerary axial canals belong to a neighbouring sexra- 

 diate, and usually also that they do not reach the centre of the 

 axial cross. Other irregularities are caused by individual 

 rays bending or changing their direction, when the two arms 

 of one axis no longer run in a straight line. 



The spot where the axial canals cross (that is to say, the 

 ideal centre from which all the six arms of a spicule radiate), 

 and where the siliceous tubes meet together, is always indicated 

 by an enlargement, the " crossing-node." 



For the classification of the Hexactinellida the constitution 

 of the crossing-nodes furnishes important data. Two modifi- 

 cations occur. 



1. The crossing-nodes form a simple stronger or weaker 

 thickening around the enclosed sexradiate axial cross of the 

 central canals [Farrea^ Aphrocallistes^ Craticularia^ Poro- 

 spongia, &c.). 



2. The crossing-nodes have the form of an open octahedron. 

 This peculiar structure is produced by the siliceous secre- 

 tion of the syncytium taking place in smaller quantity at the 

 crossing-nodes. The central canals of the six rays form an 

 axial cross enclosed by very thin tubes in a hollow octahedral 

 space, bounded by oblique siliceous beams, by which the six 

 thickened arms of the coalescent sexradiates are united. Of 

 such oblique uniting l)cams there are always twelve arolmd a 

 crossing-node ; and they are always placed exactly like the 



