— 51 — 
some instances several different genera have the mesh spicule 
so nearly alike that it is impossible to determine to which 
of the group the detached spicules may belong, in other cases 
the spicules belonging to a single genus are very clearly dif- 
ferentiated. Professor Zittel has divided the order of Lithisti- 
dae into four Families according to the structure of the spi- 
cules of the mesh and two of these families are present in 
the Horstead flint. 
Family Megamorina, Zittel. 
The Skeleton spicules of this group are comparatively 
large, elongated, oftentimes branched bodies, mostly very irreg- 
ular in their form. They are united together to form the 
skeletal mesh either by having the concave expansions of the 
terminal arms of the spicules, closely fitted against the arms 
of adjoining spicules as in the Genus Lyzdium, O. Schmidt ; 
or by the intertwining of the extremities of the spicules round 
each other as in the case of the genera Carterella, Zittel 
and Jsoraphinia, Zittel. 
Genus Lyidium, O. Schmidt 1870. 
Lyidium Zitteli n. sp. 
(Plate IV, figs. 1—9). 
Robust elongated spicules, simple or branched, of various 
forms, with the extremities of the arms transversely expanded 
and concave, so as to be attached to the surfaces of adjoin- 
ing spicules. Average diameter of the arms of the spicules 
O;412..mm. 
There is such an extraordinary diversity in the forms of 
the spicules of this species, that in some hundreds of examples 
which I have seen, I do not think two similar ones could be 
found. The simplest form is a curved spicule with the two 
ends transversely expanded (fig. 2); from this, the next stage 
is a curved spicule with a projecting arm in the centre of 
4* 
