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 Lyidium , 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 Isoraphinia, 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, 1 1 2 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 



