354 Mr. W. J. Sollas on the Genus Catagma. 



It is a matter of great regret to me that otlier duties have 

 precluded me from finishing this work during tlie past year ; 

 but as Professor Zittel is about to publish another of his ex- 

 haustive monographs on the fossil sponges, this time dealing 

 with those characterized by a reticulate calcareous skeleton, it 

 seems, on the whole, best for me to publish at once the obser- 

 vations I have already made witliout waiting an indefinite 

 period longer for fresh facts to come to light. 



On examining a thin slice from Manon macropora, margi- 

 natum^ or porcatum, one will observe an irregular network of 

 anastomosing calcareous fibres, with rounded meshes filled up 

 Avith crystalline calcite, the crystals of which radiate from the 

 sides of the fibre towards the centre of the mesh. The edges 

 of the fibre are usually coated with an opaque, granular or 

 fluffy material suspended in the adjacent calcite ; and the 

 minute interstices between the infilling crystals of calcite are 

 usually occupied by an insoluble yellowish-coloured mineral 

 which has no action on polarized light. The fibres, under a 

 magnification of 145 diameters, are found to consist of a 

 brownish-coloured calcite, often fibrous in appearance, con- 

 taining a number of spicular forms of two kinds, the most 

 numerous of whicli are slender thread-like forms, 0'0003 inch 

 to 0*0004 inch broad, but of indeterminate length, since in no 

 single instance has a perfect spicule showing both ends been 

 observed : the longest measured portions attain a length of 

 0'012 inch ; how much longer an entire spicule would be we 

 have no means of judging. 



These spicules are sometimes straight or nearly so (PL XIV. 

 fig. 5), but more usually bent, cither in a gentle curve parallel 

 to the curvature of the fibre (PI. XIV. fig. 2) or in several 

 curves so as to become undulating (PI. XIV. figs. 3 & 4). 

 Sometimes they are abruptly bent in somewhat angular turns 

 (PI. XIV. fig. 8). They are not so crowded together as in 

 Pharetrospongia, but, lying further apart, somewhat resemble 

 a number of pieces of thread floating in a viscid medium. 



The second kind of spicules is indicated by sections which 

 have very different shapes according to the direction in which 

 they traverse the spicule. The simplest shape of all is a 

 circular space (PI. XIV. fig. 9) -which is filled with colourless 

 transparent calcite, and is of course a transverse section through 

 a more or less cylindrical shaft. In the centre of this circular 

 section there is very frequently visible a minute opaque spot, 

 which appears black by transmitted -light ; it possibly repre- 

 sents the axial canal of the spicule. Transverse sections 

 through the uniaxial spicules would have a similar form ; but 

 the two, independently of other differences, can be often 



