SPONGIDA. 



145 



this skein of " anchoring fibres," there are branched spicules, 

 which are four-armed or five-armed in the recent forms, but 

 some of which in the fossil forms are hexradiate. A Car- 

 boniferous species has been described as Hycdonema Smitliii ; 

 and its " rope " was long known to palaeontologists under 

 McCoy's title of Serpula parallela, being supposed to be 

 formed of the parallel tubes of one of the Tubicolous 

 Annelides. The root -fibres of Hyalonema Smithii are of 

 large size, with a minute central canal, and terminating 

 in anchoring booklets, their sides being smooth. Silurian 

 species of Hyalonema, still undescribed, are also known to 

 occur. 



Nothing is at present known of Permian Hexactinellids 

 (unless we refer here the imperfectly examined Bothroconis 



Fig. 40. Portion of the skeleton of Tremadictyon reticulatitm, enlarged 50 diameters, from 

 the Jurassic (after Zittel). The original spicules are soldered into a continuous trellis-work 

 by a coating of silica ; but their position and hexradiate form is shown by their axial canals. 

 The " crossing-nodes," or points of intersection of the arms of each spicule, are solid. 



of King). No members of the group, also, have been 

 hitherto detected in the Trias, and they are scarce or 

 wanting in the lower portion of the Jurassic series. In 

 the Tipper Jurassic, however, we meet with a great number 

 of Hexactinellid Sponges. Of these, Craticularia, Verru- 

 cocodia, Tremadictyon (fig. 40), Sporadopyle, and Sphenaulax, 

 possess a skeleton built upon the same type as the living 

 VOL. I. K 



