456 THE VOYAGE OF H.M S. CHALLENGER. 



CO idea ■\vliicli Ehreuberg united in his group Litliocyclidina (1875, represented by four 

 genera and eight species) ; several of these, however, appei-tain to cpiite different families, 

 as his Astrom.ma entomocora, LithocycUa amjihitrites, &c. His knowledge of the 

 structure was very imperfect. The peculiar differentiation of the genera and species 

 exhibits the greatest analogy to that of the following famil}-, Porodiscida, thougli the 

 structure of the central disk in both families is quite different. 



The Phacoid Shell, or the circular, lenticular cortical shell exhibits in the Cocco- 

 discida quite the same structure and composition as in the Phacodiscida, described above 

 (p. 420), so that there can be no doul)t as to the phylogenetic origin of the former from 

 the latter. Quite in the same way in both families, the lenticular " phacoid shell " is 

 connected by numerous, short, radial beams with the intracapsular, simple or double, 

 medullary shell ; and also here these beams are commonly disposed in two groups around 

 the poles of the shortened main axis of the lens, so that their distal ends are implanted 

 in both circumpolar areas (PL 36, figs. 2-6 ; PI. 37, figs. 3, 7 ; PI 38, figs. 2, 7). 

 Tho, medullary shell is commonly simple, spherical, sometimes a little lenticularly com- 

 pressed ; more rarely it is doulile, composed of two concentric lattice-shells, which are 

 connected by radial beams ; in this case either both concentric medullary shells are 

 spherical, or the inner is spherical, and the outer lenticular, ver}' rarely the inner is lenticu- 

 lar also. In average size and structure they agree f)erfectly with those of the Phacodiscida. 

 T7ie Chamber Girdles or " chambered rings " around the equatorial margin of the 

 disk, which constitute the only difference between the Coccodiscida and the Phacodiscida, 

 seem to exhibit a considerable degree of difference of structure in the numerous 

 species of this family ; but I regret that I cannot ex23lain them here satisfactoril}'. The 

 study of these structures is extremely difficult because of the thickness and darkness of 

 the massive opaque shells ; to get a perfect knowledge of them, it is indispensable to 

 compare slides made in different directions (horizontal slides through the equatorial and 

 parallel planes, vertical slides through radial and parallel planes, oblique slides in 

 different directions). But this requires a long time and a most careful study of the 

 slides, which are very difficult to get in satisfactory condition. Therefore the fdllowing 

 remarks can have only a provisional value. 



In all Coccodiscida we can distinguish on the equatorial chamber-girdle of the lens- 

 margin (even on superficial inspection) three difterent elements of structure, viz.: — 

 (A) concentric circular rings in the equatorial plane ; (B) numerous ladial l^eams pierc- 

 ing the former and dividing them into im2:)erfect chambers ; (C) porous plates or sieve- 

 plates on both convex faces of the disk. The probable morphological significance of 

 these three elements is the following : — Each ring or girdle corresponds to an outer 

 lenticular cortical shell, which is only developed on the marginal part, whdst its central 

 part is represented by the phacoid shell. Therefore the radial beams (separating the 

 imperfect chambers) are the same as in the concentric Polysphserida , and the sieve- 



