REPORT ON THE RADIOLARIA. 561 



Family XXII. P y l o d i s c i d a, u. fam. (PL 48, figs. 12-20). 



Definition. — Discoidea witliout pliacoid shell, witli flat discoidal shell, in which 

 a simple spherical central chamber is surrounded by one or two concentric trii-adial 

 girdles ; each giixUe with three gates, separated by three simple arm-chambers. Surface 

 of the disk with three open or latticed gates on each flat side. 



The family P y 1 o d i s c i d a represents a new small but interesting group of 

 Discoidea, which exhibits rather complex affinities to different groups of 

 Sphser ell aria. In my Prodromus (1881, p. 464) I had enumerated only two 

 genera of this family, Trio}^yle and Hexapyle, and had united them with Tetrapyle 

 and allied genera in the family Pylonida. Indeed, the resemblance of skeletal structure 

 in the two groups is very great. The most simple forms of both groups exhibit a simple 

 spherical latticed central chamber, which is surrounded by few latticed chambers of 

 similar size and form, separated by open gates. But in the Pjdonida these chambers 

 are opposite in pau-s, and form together a complete lattice-girdle around the central 

 chamber, whereas in the Pylodiscida the chambers are not opposite in pairs in one 

 axis, and form therefore only latticed half girdles, which arise from the central chamber 

 like radial arms, and may perhaps better be called " arm-chambers " ; their number is 

 constantly three. The free open spaces between these three arm-chambers form three 

 gates, comparable to the two or four gates of Amphipyle, Tetrapyle, &c., and become 

 afterwards closed by lattice-work in a similar way in both groups. A more important 

 difference between them is indicated by the further mode of growth. The Pylonida 

 ibuild new girdles in all three dimensive planes (alternating in the transverse, lateral, and 

 sagittal planes) ; their geometric fundamental form is therefore the " lentellipsis " or the 

 " triaxial ellipsoid." The Pylodiscida, however, grow only at the peri23hery of the 

 discoidal shell in one single plane (the equatorial plane) ; their fundamental form is 

 therefore the biconvex lens or the flat disk (a shortened cylinder). This important 

 difference is my deciding motive, in separating the latter from the former and in regard- 

 ing the Pylodiscida as true Discoidea, the more so as they can easUy l)e derived 

 from Archidiscus, the fundamental and ancestral form of the Porodiscida. 



One single form of Archidiscus seems to be of peculiar importance in this relation, 

 viz., Archidiscus hexonicxis (PL 48, fig. 10). In this species the simple central chamber 

 is surrounded by a latticed ring or girdle, composed of six ecj[ual chambers of the same 

 size and form, all lying in the same plane with the central chamber. In a nearly allied 

 species, viz., Archidiscus pyloniscus, the six ring-chambers are different, three smaller 

 (with denser network) alternating with three larger (of looser network) ; if we imagine 

 the network of the latter reduced to a marginal bar we get Triopyle, and if also this 

 bar disappear by reduction we get Triolena, the most simple form of the Pylodiscida, 



(ZOOL. CHALL. EXP. — PART XL. — 1885.) Rr 71 



