f>08 Mr. R. Kirkpatrici on 



with rays united by cement into a firm reticulum with ovoid 

 or sometimes rectangular meshes about "19 X 14 mm. in total 

 diameter, the spaces being *095 x "057 mm., and the strands 

 on an average *047 mm. thick. The cement covers the 

 whole spicule, which can be dimly discerned in the axes of 

 the strands of the network ; occasionally it is possible to 

 observe definite orientation, all the odd (gastral) rays of 

 spicules pointing in one direction. 



The cement commonly shows a fibrillar structure, the 

 fibrillar radiating from axis to periphery of a strand and pro- 

 jecting more or less beyond the surface of a common matrix 

 in which they are imbedded, thereby dulling the vitreous 

 transparency. 



In the axis of the strand of a mesh of the network can be 

 seen the ghostly homogeneous ray of the quadriradiate, 

 which is often provided with a (ew conical spines. I at first 

 thought the fibres might be separate scleres, but the key to 

 the structure of the cement was found by discovering place* 

 where the material was beginning to be laid down, in such 

 places (PI. XV. fig. 9) a thin 61m is seen spreading over the 

 .surface of a ray ot a quadriradiate ; at and near the edge of 

 the film the surface is smooth, then granular; later the 

 granules have become tubercles, which gradually increase in 

 length till they resemble long slender cones like pointed 

 stalagmite pillars on the floor of a cavern ; still later the spaces 

 between the pillars become filled in by the deposition of more 

 matrix, and the nipple-like points of the conules project above 

 the suiface. Sometimes the cement is laid dovui in flakes, 

 and these, too, are nipple-pointed at the periphery. 



A stiucture showing radiating fibrillae is present in many 

 fossil Pharetron sponges. Zittel (7. p. Gl, fig. 18, and 6. 

 pi. xii. fig. 5) attributes appearances of this kind, in some in- 

 stances, to the effect of mineral changes, and perhaps they may 

 be ; his fig. 88 (I. c. supra) is explained, " Fasern eines 

 fossilen Kalkschwammes durch Kristallisation verandert"; 

 but the investigation of the cement of the firm skeleton of 

 Minchinella, and still more of Merlia, to be described below, 

 leads me to think that the fibrillar appearances in the fossil 

 Pharetrones above referred to may be due to the vital activity 

 of the sponge. 



One of the decalcified stained sections had some small 

 fragments of the skeleton still remaining undissolved or only 

 partially dissolved. Here it was possible to see the cells 

 whose function it is to secrete the cement. 



By way of analogy with the term " spongoblasts," the 

 name given by F. E. Schulzc to the spongin-secreting cells 



