M. K. A. Zittel 071 Fossil LitJdstidce. 131 



ments, but of elegant and sometimes minute siliceous corpus- 

 cles, which are remarkable for their irregularly branched, 

 many-pointed form (fig. 6). The slender, curved branches 

 are either similarly developed, or one of them appears as the 

 main stem in consequence of its greater length and strength, 

 and from it the others are given off as secondary branches. 

 The main stem and the branches are always set with root-like 

 or wart-like simple or forked lateral processes. These filigi-eed 

 corpuscles, from which I name this group Rhizomorina, not 

 unfrequently fork into four principal arms, and then remind 

 one of the Tetracladina ; but it is very rarely that the four 

 branches meet together at angles of 120°. In general, it may 

 be said that no general law can be established for the Rhizo- 

 morina with respect to their ramification ; they are irregularly 

 formed, and show a definite typical form only within the same 

 genus and species. 



The presence of an axial canal is generally difficult to ascer- 

 tain, as the round, toothed branches usually appear quite solid 

 by transmitted light. But with favourable preservation and 

 illumination I have been able to observe an axial canal in both 

 recent and fossil Rhizomorina. In the Jurassic forms Hyalo- 

 tragos (PL VIII. fig. 6), Platyclionia^ and Cnemidiastrumt\\Qx& 

 is in the main stem a short, straight, simple axial canal closed 

 at both ends ] in the Cretaceous and recent Rhizomorina the 

 wide, indistinctly limited axial canal, which sometimes shines 

 through like a somewhat brownish nuclear stripe, follows the 

 course of the main stem, and usually also sends ramifications 

 into the larger branches, the smaller branches and the root- 

 like processes being perfectly solid. O. Schmidt has figured 

 such axial canals in CoraUistes microtuherculatus [l. c. pi. iii. 

 fig. 4) and CoraUistes typus [L c. pi. iii. fig. 3). Among 

 the fossil Lithistidge the genera Seliscothorij Chonellaj Verru- 

 cuh'na, &c. show the axial canals distinctly. 



In the arrangement and union of these little toothed skeletal 

 elements great variety prevails. Sometimes the fine processes 

 of neighbouring corpuscles interlock to form a loose confused 

 tissue, which, when treated with acid, either breaks up into 

 its constituent particles, or sometimes remains loosely con- 

 nected ; or they group themselves close together and form 

 anastomosing or parallel fibre-like bands, in which the parti- 

 cles, which are generally deposited in definite directions, are 

 very intimately interlocked by their branches and root-like 

 processes. 



In the isolated siliceous structures which lie partly at the 

 surface and in part scattered in the skeleton, and which are 

 denominated " suri'ace-spicules " and " flcsh-spicules," greater 



9* 



