14 Mr. W. J. Sollas on Stauronemaj a new 



covering over the oscular opening, and giving ofl" one or two 

 pendent ])rocesses into the excurrent canal ; and, moreover, the 

 skeletal libres which surround the canal are also produced 

 into outgrowths of delicate reticulation and irregular fibres 

 which straggle across the canal from side to side (PI. III. 

 fig. 2) ; and the tendency of the fibre to pass into secondary- 

 growths thus manifested is carried so far that, even in the 

 normal smooth network not immediately surrounding the canal, 

 an occasional spine puts in an unexpected appearance. 



The superficial network does not frequently occur over the 

 posterior mass ; and its rarity in this position ap])ears, in some 

 cases, to result from the wear and tear to which a convex 

 surface like that of the posterior mass is especially ex- 

 posed ; in other cases it is due to a less favourable state of 

 fossilization than obtains in the anterior plate ; while in others 

 still it would appear to be absent because the posterior surface 

 has never been furnished with it, which last, indeed, is only 

 what one would expect on the view that the posterior mass is 

 an aftergrowth which increases behind while the aftergrowth 

 of fine network is extending itself in front. It is only when 

 the posterior mass has, like the attached anterior plate, ceased 

 to grow, or, at all events, when its growth has for a time been 

 arrested, that one would expect to find a final overgrowth of 

 fine network on its surface. Such a layer I have met with in 

 one case only, though whether it is, in this particular instance, 

 exceptionally produced or exceptionally preserved, is of course 

 impossible to say. This network, under a magnifying-power 

 of 50 or 60 diameters, appeared to be without a sexradiate 

 arrangement, its meshes not having any very regular form, 

 and each of its fibres seemed to be pitted or perforated with a 

 number of minute holes (PL V. fig. 1) ; but when a power 

 of from 100 to 140 diameters was applied, it was found that 

 these minute holes were the intermeshes of a delicate net, and 

 that each fibre was itself a complex reticulation of exceedingly 

 delicate fibrelets (PI. Y. fig. 2), which, wdiere most per- 

 fectly preserved, showed a regular sexradiate disposition, with 

 nodes distant yvVu to ■jtoViJ i'lch from each other, and fibre 

 Wiiu to ToVo '^^^^^ in diameter. Where a sexradiate arrange- 

 ment could not be detected, the defect appeared to be owing 

 to the disappearance of some of the fibrelets necessary to the 

 arrangement, by solution or otherwise. The cylinders of 

 network exhibit sometimes a central axis of solid fibre from 

 which the finer rete is given off all round ; and sometimes they 

 pass into a solid fibre ornamented with projecting fibrelets — a 

 transformation apparently due to the fusion of the compound 

 network-fibre into a solid one by the fuither deposition of 



