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



and Cyrtoidea. Therefore we encounter here for the first time that characteristic 

 " cortinar structure " which is complete in Cortina and Cortiniscus (PI. 92, 

 figs. 1113, 21), and which may be regarded as the strongest argument for a close 

 relationship, or even for a common monophyletic origin of all NASSELLARIA. 



The four spines, which we regard therefore as " cortinar spines," exhibit a twofold 

 kind of central junction. In the simpler case they are united in a common central 

 point, on which rests the porochora of the central capsule (Plagoniscus and Plectaniscus, 

 PL 91, figs. 4, 9). These forms are nearer to Cortina, and may be derived immedi- 

 ately from Tetraplagia and Tetraplecta by differentiation of the four equal spines. In 

 the other case the four cortinar spines are separated in pairs, diverging from the two 

 poles of a short horizontal common " central rod " (Plagiocarpa and Periplecta, 

 PI. 91, figs. 5, 10). These forms may be compared with the spicula of some 

 Beloidea and derived from Plagonidium ; but their basal central rod may be 

 compared again with the basal part of the sagittal ring of Cortina, and this comparison 

 becomes. very important in those forms like Plagiocarpa procortina (PI. 91, fig. 5). 

 Here the four spines approach very nearly to those of Cortina; the two ventral spines 

 (or pectoral feet) on the anterior pole of the middle rod are equal, but very different 

 from the two dorsal spines, arising from the posterior pole ; the lower odd spine of the 

 latter corresponds to the " caudal foot," the upper spine to the " apical horn " of 

 Cortina and of the Cyrtellaria. The vertical plane, determined by these two 

 dorsal spines, is the sagittal plane, and two opposite curved branches which lie in this 

 plane (an upper arising from the basal part of the apical spine and a lower arising from 

 the anterior pole of the middle rod) may be regarded as ventral parts of an incomplete 

 sagittal ring. This interesting form and some other similar Tetraplagida may be 

 regarded either as beginning Stephoidea (Cortina, with incomplete sagittal ring) 

 or as retrograde Stephoidea (Cortina, with partly reduced sagittal ring). In every 

 case they seem to indicate the near relationship between the Stephoidea and 

 Plectoidea. 



Another argument for this close relationship may be found in the position of the 

 central capsule in the interesting genus Plagiocarpa (PI. 91, fig. 5). Its basal part 

 (with the porochora) rests upon the common central rod, its ventral face upon the ventral 

 prolongation of the latter, its dorsal face upon the apical horn ; its axis lies in the 

 sagittal plane. The three basal spines (the odd caudal and the paired pectoral feet) 

 diverge from its basal pole downwards in the same manner as in the Cortinida, the 

 Zygospyrida and the Monocyrtida. 



Less important than those quadriradial Tetraplagida and Tetraplectida, are the 

 sexradial Plectoidea, the Hexaplagida and Hexaplectida. These may be derived 

 immediately from the triradial Plectoidea by prolongation of the three primary 

 original spines (of Plagiacantha) over the common central point. Here also two 



