_ 5 6 - 



Genus Plinthosella, Zittel 1878. 



Plinthosella squamosa, Zittel. 



(Studien itber fossile Spongien p. 153, Taf. 10, fig. 5). 



(Plate IV, figs. 3546.) 



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 - m 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 



