BY R. VON LENDENFELD, PH.D. 497 



little patches of a very dense net work are observed in the 

 proximity of the main fibres. The meshes in these are very 

 irregular and average a diameter of 0*02 mm. 



The ordinary meshes are likewise very irregular in shape and 

 size, and more regular where the main fibres are far apart, than 

 where they are close together. Their size varies O'l to 0'22 mm. 



The connecting fibres are remarkable for their small diameter. 

 The thickest, which form about 50% of the whole measure 0-024 

 mm., whilst 40% are formed of fibres averaging O'Ol mm. in 

 thickness. The remainder are still more slender fibres, the 

 thinnest observed by me in this variety measured 0-004 mm. 



IV. EUSPONGIA IRREGULARIS JaCKSONIA. 



The main fibres are cored with a dense mass of large sand 

 grains. These, however, are only found in the central part of the 

 fibre, so that its surface is perfectly smooth. 



The main fibre has a thickness of 0-1 mm. 



The sand-grains measure on an average 0-05 mm. All the 

 sand-grains seem to be of uniform shape, and also very similar in 

 size. They are all more or less spherical. 



The connecting fibres express the peculiar irregularity of the 

 fibres in this sub-genus, in the most striking manner. 



It is diflicult to give measurements. The meshes are of similar 

 shape and size in all parts of the sponge. Square ones predominate. 



They measure from 0-07 to 0*17 mm. The size of the meshes 

 is in proportion to the size of the fibres. 



The fibres vary very much in thickness. 



20% of the fibres are very thick, 0-04 mm. in diameter. Fibres 

 with a thickness of 0-01 mm. form about 30% of the skeleton ; 

 "whereas the remaining 50% is formed of fibres which measure 

 only from 0-003 — 0-008 mm, in thickness. 



The mutual relationship of these varieties to each other, and 

 to the other sub-genera is shown in the following table. If we 

 assume, as is usual in such cases that the connecting links, the 



