REPORT ON THE RADIOLARIA. 1715 



ventral horn larger than the dorsal (PL 124, figs. 3, 6). Rarely the two caudal horns 

 are fenestrated at the base and reach a considerable size, as in Conchoceras (PL 124, 

 figs. 15, 16). 



The walls of the bivalved shell usually exhibit in the Concharida a rather solid 

 shape and regular structure, with an elegant network of regularly arranged pores. But 

 in some species the walls of the shell become very thin and fragile, and assume the 

 same shape .(with very irregular network), as in the Ccelodendrida and Coelographida. 

 The pores are usually small and numerous, circular, often hexagonally framed (PL 125, 

 figs. 4-6). They pierce the thick shell-wall either in a radial or in an oblique direction. 

 Sometimes each pore is armed with six radial teeth (PL 123, fig. 7a). At other 

 times each pore represents an oblique ampullaceous canal, dilated in its middle part, 

 with two narrow openings (PL 125, figs. 5 a, b, c, 6). The pores are so arranged in the 

 majority of species that they form regular curved series, which are separated by 

 prominent crests, and converge towards the poles of the main axis. Usually the 

 marginal pores (along the frontal margins of the valves) are much smaller (compare 

 Pis. 123-125). 



The central capsule of the Concharida, very well preserved in numerous specimens 

 of the Challenger collection, constantly possesses the same situation and structure. It is 

 always enclosed in the aboral or posterior half of the shell-cavity, whilst the oral or 

 anterior half is filled up by the phseodium. The free spaces between both and between 

 the inner surface of the shell are completely filled up by the jelly of the calymma, which 

 also covers the whole shell as a thin outer jelly-envelope. The form of the central 

 capsule is sometimes nearly spherical, usually somewhat compressed in the direction of 

 the main axis, and sometimes also in the direction of the frontal axis (PL 123, 

 figs. 19). In some species it becomes bilobed, with an upper dorsal and a lower 

 ventral lobe, and in some others it becomes triangular (PL 125, fig. 7). Its two 

 membranes (inner and outer) are often separated by a broad colourless interval, contain- 

 ing a clear fluid or jelly (PL 123, figs. 8, 9). The nucleus is usually about half as 

 large as the central capsule and ellipsoidal, its longer axis lying in the sagittal diameter 

 of the body. Several specimens (of different genera) contained two separate nuclei, one 

 placed in the dorsal, the other in the ventral half of the capsule (PL 124, fig. 6). 

 This duplication of the nucleus is probably the preparation for the division of the 

 capsule. The division will be probably effected in the frontal plane, so that each half 

 of the bisected capsule gets one nucleus and one valve, and the other valve becomes 

 newly formed (in a way similar to that in the bivalved Diatomacese). The astropyle, or 

 the main-opening of the capsule, is closed by a radiate operculum, from which arises a 

 tubular proboscis ; this lies in the main axis of the body, is directed towards the anterior 

 mouth of the shell, and surrounded by the phseodium. The two shorter tubes of the 

 paired parapylse, or the accessory lateral openings, lie on the posterior or caudal side of 



