1592 THE VOYAGE OF H.M.S. CPIALLENGEE. 



more than 0-03 or less than 0-005 mm. Their surface is either smooth or slightly 

 spinulate. Examined in the dry state and by strong lenses, they constantly exhibit a 

 fine, but distinct longitudinal striation, as the expression of concentric stratification. A 

 fine axial canal or central tubule is usually visible in the axis of each rod, and often this 

 axial canal is studded with numerous short lateral branches (PI. 107, figs. 2, 4, 8). But 

 very often the axial canals become rudimentary or lost, or are developed only in a part 

 of the bars (PI. 12, fig. 1). When they are completely developed, the entire network 

 of the shell is drained by a reticulate system of communicating axial canals ; they are 

 probably filled by jelly in the living body. This tubular system never attains that regu- 

 larity and high development which is constantly found in the Aulosphserida ; and the 

 characteristic stellate nodal points of the latter, with their astral septa, are never found in 

 any of the Orospheerida. Moreover the wall of the tubular bars is very thin and struc- 

 tureless in the Aulosphserida, very thick and stratified in the Orosphserida, the enclosed 

 canal very wide in the former, very narrow in the latter. The stratification of the con- 

 centric cylindrical lamellae, which surround the narrow axial canal, is eftected by the 

 gradual dejiosition of the concentric layers, and is very similar to that which is found in 

 the thick spicula of many sponges. The peculiar structure of the bars in the Orosphasrida 

 becomes very distinct if the skeleton be burned, or acted upon by fire for some time ; it 

 then assumes a brown colour and its surface often appears dimpled. Sometimes the 

 concave dimples on the surface of the bars are rather deep and separated by prominent 

 crests (PI. 107, figs. 4, 7). The few genera of Orospha^rida which are here 

 distinguished have all the same structure, are closely allied, and diiFer mainly in the 

 shape of the outer surface of the lattice-sphere. The latter is quite simple and smooth 

 only in Orona. In the common Orosphcera (PL 106, figs. 1-3) it is studded with 

 radial spines. In the most frecpient form, Oroscena, the shell has a pyramidal or 

 tent-shaped elevation on the base of each radial spine, and usually the bases of the 

 neighbouring spines are connected by strong jjromineut crests, the edges of the three- 

 sided or four-sided pyramids (PI. 12, fig. 1 ; PI. 106, fig. 4). This remarkable form 

 is more or less polyhedral, with concave sides, and is similar to the characteristic shells 

 which are represented by Aidoscena among the Aulosphserida, by Sagoscena and 

 Sagoplegma among the Sagosphserida (compare Pis. 108 and 110). The pyramids or 

 tents, however, are in these latter more regularly and distinctly developed than in the 

 Orosphserida. The radial spines which arise from the top of the pyramids are often 

 branched, and the branches become connected to form an outer enveloping secondary 

 shell or a loose spongy framework in Oroplegma (PI. 107, fig. 1). 



The radial spines of the Orosphaerida never exhibit a constant number or disposition in 

 the individual species ; their usual number is from twenty to sixty. Their form exhibits 

 two different types, which, however, are not sharply separated ; robust club-shaped and 

 slender rod-shaped spines. The rolxist club-shaped spines are usually about as long as 



