A. The distinction between medullary and cortical shells was originally based in my Monograph (1862, p. 50) upon the topographical relation of the lattice-shells to the central capsule, inasmuch as I regarded all intracapsular shells as medullary, all extracapsular as cortical. Hertwig, however (1879, p. 122), rightly pointed out that this distinction is unpractical, "because the same lattice-shell in the same species may lie within or without the central capsule, according to the size of the latter." He proposes, therefore, to restrict the term medullary shell to the innermost, and to call all the others cortical; a course which seems justified by the special significance of the primary innermost lattice-shell ("as the point of origin of the radial spines"). But in most #Sphærellaria# which form three or more concentric shells, the two innermost, which lie near together within the {lxxxvi}central capsule, are very different in size and dictyosis from all the others which lie outside, and are separated by wider interspaces (compare Pls. 17, 24, 29-32, 40, &c.). In these cases it appears better to regard the two inner as inner and outer medullary shells, and all the others as cortical shells. The character of the dictyosis in the intracapsular and extracapsular shells is often so different that I have made it the basis of separation of _Thecosphæra_ and _Rhodosphæra_ among the Liosphærida (p. 60), of Elatommatida and Diplosphærida among the Astrosphærida (p. 208), &c. B.--R. Hertwig (1879, L. N. 33, pp. 40, 123) separates the true (simultaneously formed) "cortical shells" (_e.g._, of _Actinomma_, _Cromyomma_) from the arachnoid "siliceous networks" (_e.g._, of _Diplosphæra_ and _Arachnosphæra_) which are formed by the successive union of tangential apophyses of the radial spines. Whether this principle is right in the theory or not, it cannot be carried out practically. Compare also Pl. 25, fig. 4.