REPORT ON THE RADIOLARIA. 1533 



phaeodium, and of the plifeodella (or the peculiar dark pigment-granules composing 

 it). At the same time R. Hertwig published his olbservations on the big living Tripylea 

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

 comjiosition, and ari'angement of this excentric extracapsular pigment-body was most 

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

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



The most stiikiug peculiarity of the phaeodium, 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 phaeodium 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 

 ph^odium, in consequence of this excentric j^osition, 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 cal}Tnma. 



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

 very characteristic in the suborder P h ge o g r o m i a, or in those PH^aEODARiA 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 phaeodium in 

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

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

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

 In this suborder (the Phaeogromia), the volume of the phaeodium may be, on an 

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

 of other Ph^odaria it is much greater than the latter. 



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

 in the P h ae o g r o m i a, also exists in the suborder P h se o c o n c h i a, or in those 

 Ph.eodaria, 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 parapylae (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 phaeodium is also placed here on 

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

 in the three families ofPhaeoconchia. 



