EEPOET ON THE EADIOLARIA. 805 



the shell). The distal apex is commonly simple, conical or pyramidal, rarely bifid or 

 truncate. The edges of the spines are commonly smooth, rarely denticulate or serrate. 



The apophyses, or the lateral transverse processes of the radial spines, in the Dora- 

 taspida assume the greatest variety and complexity in form, size, mode of ramification, 

 and in composition of the shell. An expert and practised observer may determine easily 

 the range of each spine, whether it be an equatorial (c), or a tropical (h, d), or a polar 

 spine (a, e. Pis. 133—138). The two opposite apophyses of the Diporaspida, as well 

 as the four crossed apophyses of the Tessarasjiida, lie constantly in certain meridian 

 planes of the spine, which have a legitimate signification for each of the five zones. The 

 comparative morphology of this regular disposition of the apophyses and the regular 

 meeting of their branches is of the greatest interest, and necessary for the complete 

 understanding of the complicated structure of these wonderful shells. 



The pores or meshes of the spherical shell, off'ering the most varied forms, may 

 generally be divided into two difi'erent groups, into sutural and parmal meshes. The 

 sutural pores are bordered by the meeting branches of the apophyses of two, three, or 

 four neighbouring spines, and therefore also by the sutures in which they meet. The 

 parmal pores on the other hand are bordered only by the united branches of the 

 apophyses of a single spine and pierce the shield or lattice-plate formed by 

 them. Therefore the shell-meshes of the C 1 a d o p h r a c t a are all sutural pores 

 (PL 137, figs. 1—8 ; rarely and only in a part of the sj^ines parmal pores also : 

 ZonaspiSj Dodecaspis) ; whereas the shell-meshes of the P e 1 1 o p h r a c t a, piercing 

 the shields or lattice-plates of all twenty spines, are alw^ays partly sutural, partly 

 parmal pores (Pis. 135, 136, 138). The parmal pores again may be divided into two 

 difi'erent groups — aspinal and coronal pores. Aspinal pores ("ad spinani") are those 

 which lie immediately on the sides of the radial spine and are bordered by the primary 

 l)ranches of its apophyses ; therefore constantly only two in the Diporaspida, four in 

 the Tessaraspida. Coronal pores on the contrary are those which lie in the 

 periphery of the lattice-plates, surrounding in a circle or crown the aspinal pores 

 and not touching the spine itself. In Dorataspis, Ceriaspis, Tessaraspis, Lychnaspis, 

 &c., all parmal meshes are only aspinal pores (PL 135, figs. 2—5 ; PL 136); whilst in 

 Coscinaspis, Acontaspyis, Icosaspis, Hylaspis, &c., one part of the parmal pores is 

 aspinal, one part coronal (PL 136). The number, form, and size of the coronal pores is 

 very variable and often very large (soinetimes more than a hundred in one plate). 



The C 1 a d o p h r a c t a exhibit a comparatively simple shell-formation ; either all 

 twenty spines or at least a part of them not forming lattice-plates. The most primi- 

 tive form among these is Phractaspis (PL 137, figs. 1, 2). The forty apophyses of 

 its twenty spines are simply forked, and then- eighty fork-branches united by forty 

 sutures, enclosing twenty-two sutural meshes : two square polar meshes (between the 

 four polar spines on the poles of the spineless axis, a a a a and e e e e); eight triangular 



