420 THE VOYAGE OF H.M.S. CHAl.LENGEE. 



for the separation of subfamilies. The form of the medullary shells is commonly quite 

 spherical (as also in the above mentioned S p h se r o i d e a) ; sometimes it is a little com- 

 pressed in the same direction as the enclosing lenticular phacoid shell. The diameter of the 

 latter is commonly three to four times as large as the diameter of the medullary shell ; if 

 this be double (in the Thecodiscida) then the diameter of the outer medullary shell is 

 commonly three to four times as large as that of the inner. The radial beams connecting 

 the two seem to be very variable in number and disposition (compare PI. 31, fig. 8 ; 

 PI. 32, figs. 3, 4a, 7, 8a ; PI. 33, figs. 2, 3 ; PL 35, figs. 4, 8, 9, &c.). 



The Radial Beams, which connect' the medullary shell with the cortical or "phacoid 

 shell," and which pierce the membrane of the lenticular central capsule, are commonly 

 aggregated into two polar bunches around the shortened axis of the disk (compare 

 PI. 31, fig. 8 ; PI. 32, figs. 3, 8 ; PL 33, figs. 2, 3, &c.). Their number seems to be 

 usually between ten and thirty. More rarely piercing radial beams lie also in the 

 ec^uatorial plane, and then commonly as inner prolongations of the outer marginal 

 spines; so we find two opposite in one axis, in HeUostylus (PL 34, figs. 1, 2), or 

 four opj)osite in pairs in two crossed axes, in Phacostaurus (PL 31, figs. 1, 2, 7). 

 Often the thickness of the beams increases from the centre towards the periphery. 



Tlie Phacoid Shell, or the lenticular extracapsular cortical shell, exhibits in the 

 Phacodiscida quite the same general character as in the Cenodiscida, above desciibed 

 (p. 410). Commonly the equatorial diameter of the lens is twice to three times as large 

 as the vertical diameter or the shortened "main axis." The convexity of both opposite 

 faces is either quite even to the sharp margin (PL 31, fig. 2 ; PL 33, figs. 2, 5), or 

 the central part of the lens is more strongly vaulted, and often the margin is thickened or 

 truncated (PL 31, fig. 10 ; PL 35, fig. 7). The surface of the lens is commonly smooth, 

 but sometimes also covered with bristle-shaped radial spines (PL 34, figs. 3, 5 ; PL 32, 

 figs. 2, 3) ; rarely these spines are prolonged and branched (PL 35, figs. 3, 5). The 

 pores of the phacoid shell are usually quite regular, circular, and regularly arranged, 

 either in more concentric or in more radial rows ; the latter are sometimes separated hj 

 radial crests arising towards the margin (PL 35, fig. G). If the wall of the phacoid shell 

 be much thickened, the pores in its central part are shorter and cylindrical, in its marginal 

 part longer and conical (PL 31, fig. 7 ; PL 32, fig. l). 



The Margin of the Lens of the Phacodiscida is very polymorphic, and serves mainly 

 for the separation of genera. In the first subfamily, Sethodiscida, it is either cpiite simple 

 (PL 35, figs. 6-8) or surrounded by a thin solid ecpiatorial girdle, the basal part of which 

 is often radially striped (PL 32, figs. 7, 8). In the second subfamily, Heliosestrida, we 

 find on the margin a small number of radial spines in the equatorial plane regularly 

 disposed, either two opposite in one axis (PL 31, figs. 9-12) or four opposite in pairs 

 in two crossed axes (PL 31, figs. 1-8), or eight opposite in pairs in four axes, crossed 

 at angles of 45° (PL 34, figs. 3, 6) ; in the latter case we can sometimes distinguish 



