the spicules are only laterally pressed together. Though I 
have not obtained specimens in which the spicules are attached 
together yet the isolated spicules so closely correspond in ge- 
neral form with the type of the genus, C. spzculigera, 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. 
