REPORT ON THE EADIOLARIA. Ixxxv 



valves are usually equal, but iu a portiou of the Coueharida they present constant differ- 

 ences. In this family the two valves are attached to each other by their free edges, just 

 as iu the bivalved Mollusca and Diatoms ; and these edges may either be smooth 

 (Conchasmida, PL 123, figs. 1-6), or dentate (Conchopsida, Pis. 124, 125); the 

 valvular connection of the latter is sometimes strengthened by a special ligament which 

 unites the two valves at the aboral pole (PL 123, figs. 8, 9). The form of the valve is 

 sometimes hemispherical, sometimes boat-shaped, with a sagittal keel. 



129. Medullani and Cortical Shells. — In all Radiolaria whose skeleton consists of a 

 double shell or of two concentric lattice-shells united by radial bars, an inner meduUary 

 shell (testa medullaris) and an outer cortical shell (testa corticalis) may be distinguished 

 (see note A, below). The medullary shell is usually to be regarded as a primary, the 

 cortical as a secondary structure. Such double shells occur among the Spumellaeia in the 

 Dyosph^rida (Pis. 1 9, 20), as well as iu many P r u n o i d e a (Pis. 39, 40), D i s c o i d e a 

 (Pis. 33, 34), and Larcoidea (Pis. 9, 10); among the Acantharia only in the family 

 Phractopeltida (PL 133) ; among the Nassellaeia only in very few Cyrtoidea {e.g., 

 Periarachnium, PL 55, fig. 11), and finally among the Ph^odaeia in the Cannosphserida 

 (PL 112) as well as in part of the Coelodendrida (PL 121) and Coslographida (Pis. 127, 

 128). In most cases (if not always?) the cortical shell arises by the growth of radial 

 spines from the surface of the meduUary shell ; these become united at equal distances 

 from the centre by transverse apophyses, the surface of the secondary calymma furnishing 

 the basis for their secretion (§ 85). Nevertheless, it seems that in many Sphserellaria 

 the formation of the whole cortical shell proceeds simultaneously (at a definite dictyotic 

 period) like that of the primary medullary shell (see note B). Whilst in the Ph^odaria, 

 AcANTHAEiA, and Nassellaria, at most two concentric shells are formed, in many 

 Spumellaeia their number increases continuously with additional growth; in many 

 S p h 86 r e 1 1 a r i a it rises to four, eight, or even more, as well as in many Discoid ea 

 (if the concentric, peripherally disposed rings of chambers be regarded as incomplete 

 flattened shells). In these cases either only the innermost primary lattice-shell is to be 

 styled " medullary shell," or at most the two innermost (inner and outer medullary shells), 

 all the others being cortical. 



A. The distinction between meduUary and cortical sliells 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 Spheerellaria 

 which form three or more concentric shells, the two innermost, which lie near together within the 



