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



Archidiscus (PL 48, figs. 911) is not only the common phylogenetic ancestral 

 form of all Cyclodiscaria, but also the common ontogenetic original form of all 

 Porodiscida, or at least of the greater part of them. The numerous species of Archi- 

 discus, which are distinguished in the sequel, are at the same time the embryonic forms 

 of different Porodiscida, corresponding to the " biogenetic main law of development." 

 The small shell of Archidiscus is sometimes completely lenticular, circular, at other 

 times more or less polygonal ; commonly on the biconvex centre much thicker than 

 on the margin, but sometimes also of nearly equal thickness (like a medal or a short 

 cylinder). The latticed central chamber of it is probably in the majority of species 

 spherical, but in some more or less compressed, lenticular ; the number of small pores 

 on its surface is probably commonly between ten and twenty (four to eight usually 

 being visible on each hemisphere). The number of radial beams, which connect it 

 with the equatorial ring, varies commonly from four to eight ; but sometimes only 

 two or three are to be found, in other cases nine to ten or more. The regular 

 disposition of these beams (in certain equatorial axes of the disk) is probably of great 

 importance, as determining the later development of characteristic radial appendages of 

 the margin in the more highly developed Porodiscida. The equatorial ring itself, forming 

 the margin of the lenticular disk, is either a simple solid ring or a broader latticed 

 girdle ; in the latter case it merges slowly into the opposite sieve-plates of the two flat 

 disk sides, or the porous " cover -plates," covering its parallel or convex surfaces. These 

 latter can be regarded as direct peripheral continuations of the polar regions of the 

 spherical central chamber. The ring-chambers, surrounding the latter in a single circle, 

 are commonly of nearly the same breadth, but often also of different irregular size. Their 

 number varies between two and ten or more, but commonly between four and eight ; each 

 ring-chamber is covered on the upper and lower side by the sieve-plate, bounded on the 

 inner (proximal) side by the wall of the central chamber, on the outer (distal) side by 

 the marginal ring, on both lateral sides by the contiguous neighbouring ring-chambers. 



The important question as to the phylogenetic origin of Archidiscus can be 

 answered in a twofold way. The most simple form of Archidiscus (Archidiscus 

 dioniscus) can be derived immediately from the Stylosphserida, Saturnalia (PL 13, 

 fig. 16), only by the development of lattice-work between the equatorial ring and the 

 two polar faces of the concentric central chamber (on the surface of the biconvex jelly- 

 mantle). But on the other hand Archidiscus may also be derived from the simplest 

 Phacodiscida, Sethodiscus (PL 33, figs. 13), by the stronger compression of the 

 biconvex lenticular shell, so that the enclosed medullary shell on the two poles runs 

 together with the lenticular phacoid shell, of which only the peripheral part remains 

 free, and thus forms the chambered ring ; this latter explanation seems the more 

 natural in many cases, as often in the Porodiscida the central chamber is enclosed 

 in one or two concentric spherical or lenticular lattice-shells. 



