REPORT ON THE RADIOLARIA. 1533 



phseodium, and of the phseodella (or the peculiar dark pigment-granules composing 

 it). At the same time R. Hertwig published his observations on the big living TBJPYLEA 

 examined by him at Messina, and pointed out particularly that the constant presence, 

 composition, and arrangement of this excentric extracapsular pigment-body was most 

 characteristic, and sufficient in itself to distinguish this group from all the other Radio- 

 laria (1879, loc. cit., p. 99). 



The most striking peculiarity of the phseodium, and the most important on 

 account of its absolute constancy, is its excentric position, covering only the oral hemi- 

 sphere of the central capsule, and wholly or partly wanting on the aboral hemisphere. 

 This constant topographical relation to the capsule never fails, and may be always 

 observed at the first glance, when the body is seen from the side (the main axis of 

 the capsule being perpendicular to the axis of the eye of the observer), (compare 

 PI. 101, figs. 6, 10 ; PL 102, fig. 1 ; PL 103, fig. 1, &c.). The phseodium envelops, 

 therefore, the oral half of the capsule completely, and especially the astropyle in its 

 centre ; hence, the radiate operculum and the proboscis arising from the latter cannot 

 be seen usually before the former is removed. The general form of the entire 

 phseodium, in consequence of this excentric position, is concavo-convex (or crescentic 

 in longitudinal section), its concave face embracing the oral or anterior face of the 

 capsule, and its convex face being turned to the surface of the calymma. 



The topographical relation of the phseodium to the surrounding shell is also 

 very characteristic in the suborder Phseogromia, or in those PH^EODARIA which 

 possess a peculiar shell-mouth placed on the oral pole of its main axis. Here 

 the capsule is always placed in the aboral half of the shell-cavity, the phseodium in 

 its oral half, separating the astropyle from the mouth of the shell, both of which 

 lie in the main axis ; as in the Challengerida (PL 99), Tuscarorida (PL 10O), 

 Castanellida (PL 113), Circoporida (PL 115), and Medusettida (PL 118-120). 

 In this suborder (the Phseogromia), the volume of the phseodium may be, on an 

 average, about as great as that of the central capsule, whilst in the majority 

 of other PH^EODARIA it is much greater than the latter. 



A similar topographical relation between the phseodium and the enclosing shell, as 

 in the Phseogromia, also exists in the suborder Phseoconchia, or in those 

 PH^ODARIA, which are distinguished from all the others by the possession of a bivalved 

 shell (PL 121-128). The two valves of this curious shell, which resembles that of the 

 Brachiopoda, are dorsal and ventral, and the tripylean central capsule is always so 

 placed between them that its two lateral parapylse (right and left) lie in the frontal 

 plane of the shell, where a large frontal fissure opens between the opposed margins of 

 the two hemispherical or cap-shaped valves. The phseodium is also placed here on 

 the oral half of the capsule and surrounds its astropyle ; but it exhibits some differences 

 in the three families ofPhseoconchia. 



