444 



PROF. A. DENDT ON PONTOBOLBOS, 



its exact form no doubt depending upon the nature of the 

 surface on which it lay. One specimen (fig. 3) is irregular in 

 shape, and appears to have been formed by the fusion of three 

 growing side by side. The upper surface occasionally exhibits 

 small round pits, which may be either deep, as in fig. 1 and 

 at J) in fig. 2, or shallow and pock-like, as at j) in fig. 4. The 

 latter are especially developed near the margin. It seems 

 certain that these pits are not proper to the organism : there 

 is no canal-system in connection with them, and they are probably 

 due to some external agency, possibly to the action of parasites. 

 Their presence certainly increases the superficial resemblance 

 of the organism to a sponge. The texture, after preservation in 

 spirit, is very compact, tough and like that of indiarubber, and 

 there is a good deal of sand in the deeper layers. When cut 

 in half vertically (PI. 26. fig. 2) the organism exhibits a very 

 strongly marked, concentrically lamellated structure, although 

 the lamellae do not show the least tendency to separate from one 

 another. Even a very thick slice in spirit is translucent, and 

 shows to the naked eye that the lamellated appearance is due to 

 the occurrence of thin opaque layers in a transparent ground- 

 substance. These opaque layers evidently mark old surfaces of 

 growth, and the distance between successive surfaces varies 

 considerably, up to about 0*75 mm. 



Under a low power of the microscope a tolerably thin vertical 

 section exhibits the appearance shown in PI. 26. fig. 5. It will be 

 seen that the opaque layers consist of a deuse flocculent granular 

 substance, and that they are connected together by a coarse 

 network of similar but less dense material the strands of which 

 ramify, chiefly in a radial direction, through the intervening 

 layers of transparent ground-substance (cf. fig. 6). This floccu- 

 lent material stains fairly readily with borax-carmine, and very 

 intensely with aniline stains such as iodine-green, fuchsin, and 

 methyl-violet. I have not, however, been able to detect any 

 nuclei in it. It also stains faintly yellow with iodine, and. more 

 intensely yellow with chlor-zinc-iodide after soaking in iodine. 

 It does not give the xanthoproteic reaction with nitric acid and 

 ammonia, nor does it give the brick-red colour with Millon's 

 reagent, so characteristic of protoplasm, though both these 

 reagents acted satisfactorily upon the protoplasm of fragments 

 of Oscillarian algee included in the sections. 



Frequently irregular layers of much coarser granular material, 



