— 55 — 



the spicules are only laterally pressed together. Though I 

 have not obtained specimens in which the spicules are attached 

 together )-et the isolated spicules so closely correspond in ge- 

 neral form with the t>'pe of the genus , C. spiadigcra, Roe- 

 mer, sp. from the Upper Chalk of Hanover that there can 

 hardly rest a doubt of their belonging to it. The spicules of 

 C. spiculigera are however both longer and thicker than those 

 in the Horstead flint, and it is therefore probable that the 

 latter belong to another species. In the Hanover spicules, 

 a central canal extends along the axis of the spicule but this 

 has been obliterated in the Horstead examples. 



Family Tetracladina, Zittel, 



In this group of the Lithistidae the individual spicules 

 are mostly built upon a quadriaxial type and consist of four 

 arms radiating from a common centre. The extremities of 

 the arms are split up into small branches and twigs, and it 

 is by the complicated interweaving of these minutely divided 

 extremities that the spicules unite with each other to form 

 the skeletal mesh work. Besides these four-rayed spicules of 

 the mesh, probably all the sponges of this division are fur- 

 nished with a surface layer ot spicules, mostly of a very diffe- 

 rent shape to those of the mesh. In some instances these 

 surface spicules are delicate laminae of various forms, as in 

 the Genus Plinthosella ; in others they are trifid spicules, in 

 which the shaft has been reduced to a mere point and the 

 head rays have coalesced to form discoidal heads with irre- 

 gular borders, as in Racodiscula; whilst other surface spicules 

 resemble in form the compound trifid spicules of Geodia, and 

 are only distinguished from the zone spicules of that genus 

 by their much smaller size. As a rule the individual spicules 

 of many of the sponges of this family are very minute, and 

 their slender arms and delicate branched extremities have been 

 considerably eroded by the process of fossilization. 



