408 THE VOYAGE OF H.M.S. CHALLENGER. 



spines alternating with four smaller (interradial), indicating radii of first and second order. 

 The section of Perissacantha is much smaller, and commonly represented only by triradial 

 forms, with three spines at equal distances (120) Triactis, Tripocyclia, Tripodictya, 

 Spongotripus, &c. (PI. 33, fig. 6 ; PI. 37, fig. 5 ; PI. 42, figs. 7-9). 



Radial Arms on the margin of the disk appear in similar variety of number, form, 

 and disposition as the radial spines ; but the number is here commonly limited to from 

 two to four, rarely five to six. The arms are absent in the families Cenodiscida and 

 Phacodiscida ; in the four other families they return under similar forms. These arms 

 are direct prolongations of the disk, and exhibit the same structure, so that they may be 

 regarded both as centrifugal productions of certain radii, and also -inversely as peri- 

 pheral parts of a disk, the interjacent radii of which are reduced. The regular disposition 

 and shape of the arms, an important character for the distinction of genera and species, 

 is repeated in a quite analogous manner in the four above mentioned families, so that we 

 can distinguish the following groups A. Amphibrachida, with two radial arms, opposite 

 on the poles of one equatorial axis (the first cross axis) Diplactura, Amphibrachium, 

 Spongobracliium (PL 38, figs. 3-5; PL 44, figs. 6-11); B. Triobrachida, with three 

 radial arms ; the most important group (with all Pylodiscida) ; either all three arms are 

 equal and disposed at equal distances (Trigonactura, Dictyastrum, Rhopalodictyum, 

 PL 38, figs. 6-9; PL 43, figs. 5, 13, 16; PL 48, figs. 12-19), or a single odd arm 

 differs in size and position, and is often larger than both the opposite paired arms 

 (Rhopalastrum, Euchitonia, PL 43, figs. 6, 10, 15, &c.) ; C. Tetrabrachida, with four 

 radial arms, opposite in pairs in two crossed axes, commonly perpendicular one to 

 another, Stauractura, Stauralastrum, Spongaster, &c. (Pis. 46, 47). 



The arms are commonly simple, undivided, but sometimes also forked or branched 

 (PL 43, figs. 15, 16; PL 47). Their basal parts are either free, separately inserted into 

 the margin of the circular central disk, or they are connected by a " patagium," a peculiar 

 connecticulum, like a web-membrane, which is composed of a chambered, commonly 

 more or less spongy framework, different in texture from the lattice-work of the arms 

 (PL 38, figs. 8, 9; PL 43, figs. 9-16; PL 46). Sometimes the patagium overgrows 

 the whole shell. A peculiar modification of it appears in Stephanastrum (PL 44, fig. 1), 

 where only the distal parts of the arms are connected by the ring-shaped patagium, whilst 

 the basal parts are free; therefore open gates rest between them, like those of the 

 Pylodiscida (PL 48, figs. 12-20). 



The Central Capsule of the Discoidea is constantly discoidal, more or less 

 lenticular; in some cases more biconvex, with vaulted faces and thin margin ; in others 

 more medal-shaped, with flat faces and thick margin. In the Cenodiscida alone the capsule 

 lies freely inside the simple phacoid shell, and is separated from it by the jelly- veil. In the 

 other five families the capsule encloses the central parts of the skeleton, and is enveloped 

 by the superficial parts of it, whilst its membrane is perforated by radial beams connect- 



