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



large open spaces in the latticed discoidal shell, which reappear in a similar shape among 

 the Larcoidea in the Pylonida ( Tetrapyle, &c. ). We get the best understanding of this 

 peculiar formation if we return to Archidiscus, probably the common ancestral form of 

 all Cyclodiscaria, of the Porodiscida as well as of the Pylodiscida and Spongodiscida. 

 In some species of Archidiscus (Archidiscus hexoniscus, Archidiscus pyloniscus, &c.) the 

 small lenticular shell is composed of a spherical latticed central chamber and of a con- 

 centric equatorial girdle composed of six such chambers, either all six equal, or alternately 

 larger and smaller. This latter form is nearly identical with Triodiscus, and if we 

 imagine the lattice-work of only three ring-chambers complete, whilst that of the three 

 alternating chambers is reduced to the marginal ring, we get Triopyle, by loss of this 

 ring Triolene (a disk composed of four simple lattice-chambers, lying in one plane, three 

 radial around one central spherule). The genera mentioned form together the subfamily 

 of Triopylida. In the second subfamily, Hexapylida, the same formation is doubled ; 

 here three double arm-chambers are separated by three double spaces (two in each radius). 

 Also here the three distal spaces may be either quite open (Pylolena), or half closed by 

 the marginal girdle (Hexapyle), or quite loosely latticed (Pylodiscus}. If the margin of 

 this latter form become surrounded by a perfect chambered equatorial girdle, we get 

 Discozonium, and if this acquire a peculiar marginal ostium (surrounded by a corona 

 of spines) we arrive at Discopyle. These two latter genera form the third subfamily, the 

 Discopylida. The eight genera of Pylodiscida represent therefore a continuous phylo- 

 genetic series. 



The Spongodiscida are the sixth and last family of the Discoidea, differing 

 from the five other families in the irregular, spongy structure of the discoidal skeleton ; 

 both surfaces of the flat disk (upper and lower) are here principally covered with a rough, 

 spongy framework, whilst in the five other families they are covered by the flat and 

 smooth porous plates or sieve-plates. Nevertheless there is no sharp boundary between 

 the Spongodiscida and the closely allied Porodiscida. In these latter also the discoidal 

 shell becomes often more or less spongy (mainly in the peripheral part, e.g., in 

 Myelastrum, PI. 47); but at least the central part of the disk here remains constantly 

 covered by sieve-plates. The massive skeleton of the Spongodiscida is either of perfectly 

 irregular structure, only composed of innumerable fine branched siliceous threads, inter- 

 woven in all possible directions ; or only the outer part of the disk is composed of such 

 spongy framework, whilst the central part is more or less distinctly composed of 

 concentric chambered rings, as in the Porodiscida. These latter forms indeed exhibit an 

 immediate transition to this family, and were formerly (in 1862) separated by me as 

 Spongocyclida. Also the polymorphous shape of the disk margin in the Spongodiscida is 

 quite analogous to that of the Porodiscida. Whilst in the first subfamily, the Spongo- 

 phacida, the margin is quite simple ; in the second, the Spongotrochida, it is armed with 

 solid radial spines ; and in the third, the Spongobrachida, it is provided with two, three, 



