CARADOCIAN CYSTIDEA FROM GIRVAN. 445 



centre, with its two lateral rays bifurcating (text-fig. 51). Thus deltoids 20 and 21, 

 to right and left of the median anterior suture, are of subquadrate outline ; the two 

 deltoids 24 and 22, at the right and left sides respectively, are of the usual subtriangular 

 outline; and the posterior deltoid, 23, would naturally be of subpentagonal outline. 

 This last plate, however, occupying a wider area than any of the others, appears to 

 have been divided into three by curved sutures cutting off its outer corners, so that 

 it assumes a heptagonal outline. The right-hand portion thus cut off is larger than 

 the left-hand portion. An extended pore (?gonopore) appears to lie on the curved 

 suture separating the body of the plate from its left-hand portion ; and at the other 

 end of the plate appears to be a folded and probably poriferous elevation (? hydropore). 

 § 324. The deltoids thus rest on the plates of Circlet IV., and the sutures between 

 the deltoids underlie the five main grooves of the Subvective System (PI. IV. figs. 

 52, 54). The surface of each deltoid is excavate in the middle along the interradius. 



Fig. 51. 



Text-fig. 50. — The outline of the Periproot of Chcirocriiiiis constrictus, reconstructed from various specimens, 

 and showing its relation to the adjacent plates with their pectinirhombs and axial ridges. Note the 

 absence of pectinirhombs in the neighbourhood of the supposed rectal expansion, and compare text-fig. 45. 



TuxT-FiG. 51. — The structure of the Tegmen in Cheirocrinus constrictus, based on G187, as explained 

 in § 323. Enlarged about 4 diameters. (See further the legend to text-fig. 44, and compare with 

 text-figs. 52-55.) 



and the sides rise up towards the grooves. The sutural margins of the deltoid are 

 chamfered to form the groove, and are furnished with a rebate along which small 

 alternating cover-plates were attached. This chamfering and rounding of the margins 

 obscures the true angular outline of the deltoids as inferred from the sutural markings 

 on the internal cast, and produces a smoothly rounded or semicircular outline on the 

 adoral side. 



The form is still further modified by the branching of the grooves at their distal 

 or aboral ends, and so assumes the outline of a battle-axe head (text-fig. 52). 



v; 325. It is not possible to reconstruct every one of the grooves from the available 

 material, but it seems probable that the three anterior grooves, corresponding with 

 plates 16, 15, and 19, branched three or four times, and bore as many brachioles (text- 

 fig. 53); but that the groove corresponding with plate 18 followed the suture, cutting 

 off the adjacent right-hand corner of the posterior deltoid (23), and gave off bifurcating 

 branches on its right side, so that the number of brachioles was five (text-fig. 54). 

 The same cannot have been the case with the corresponding grooves on the left of the 



