— 56 — 
Genus Plinthosella, Zittel 1878. 
Plinthosella squamosa, Zittel. 
(Studien iiber fossile Spongien p. 153, Taf. 10, fig. 5). 
(Plate IV, figs. 35—46.) 
Spicules of the mesh robust, mostly four armed, but the 
arms unequal, straight or curved and radiating from the centre 
at different angles. The surface of the entire spicule is com- 
pletely covered with prominent rounded wart-like projections. 
The extremities of the spicular arms are frequently split up 
into small twigs, which are twined round the arms of adjoining 
spicules, and the spicular arms themselves are closely adpres- 
sed together so that the warty projections interlock with each 
other; by this means a very compact skeletal mesh work is 
formed. Length of arm of large spicule 0,75 mm.; average 
thickness 0,16 mm. ‘The surface spicules of the sponge are 
formed by delicate laminated plates of varying forms, either 
circular, (fig. 45); oval (fig. 46); straight and narrow, (fig. 43) 
or curved and tapering to both ends. The narrow plates are 
about 2 mm. in length and 0,157 mm. in width, whilst the 
circular plates have a diameter of I mm. 
The mesh spicules of this sponge are mainly characterized 
by their robust proportions, the inequality of their arms, and 
the prominent rounded knobs with which these are furnished. 
In some instances, the spicules are apparently formed but of 
three rays or arms, two of which are often in the same plane 
and at right angles to each other, other spicules have bent 
and curved arms, some as long again as others. Most of the 
arms terminate in two or three minute, irregular, twig-like 
extensions which serve to attach the spicules to each other 
to form the mesh. These twig-like extremities do occasionally 
intertwine with the similar terminations of adjoining spicules, 
but, as a rule, they grasp round the sides of the spicular 
arms, and oftentimes the arms of different spicules are, for 
