CARADOCIAN CYSTIDEA FROM GIRVAN. 



443 



casts, and thirty retain their counterparts. No specimen is complete, and even the 

 theca is imperfectly seen in most. 



§ 314. Description. — The Theca has a slightly impressed base (Circlet I., text- 

 fig. 48), whence it swells outwards to the level of the umbones of Circlet II. ; it is 

 constricted towards the summit of Circlet III., the constriction being accentuated by 

 the periproctal excavation ; and it expands slightly again towards the summit of 



Text-fig. 48. — Cheirocrinus constrictus. 



The bases of G133 and G 160, represented as nearly as possible in the same orientation, 

 viz. with the posterior interradius towards the observei', and thus manifesting the different 

 planes in which these two specimens are compressed. Enlarged about 2 diameters. 



Circlet IV., where the brachioliferous median regions of the plates project; finally, the 

 tegmen, composed mainly of Circlet V. (Deltoidea), is approximately flat. Thus in 

 general shape the theca may be roughly compared to the helmet of an uhlan or lancer 

 (PI. IV. figs. 49, 51). 



^--TS ,-«ffl#^ C2^ 



Text-fig. 49. — kwiXyais oi Cheirocrinus consirictus. The outline of deltoids 21, 22, 24, and 20 is based on 

 the sutures as seen in the internal cast of the tegmen (text-fig. 44) ; the proliferated brachioliferous 

 elements are shown only attached to deltoid 23. The arrangement of the periproctals is imaginary, but 

 their size and the outline of the area occupied by them are approximately correct. The invaginate 

 portion of the basals is indicated by a broken line. 



§ 315. The outlines of the individual plates can be grasped more easily from the 

 accompanying analysis (text-fig. 49), based on examination of a large number of 

 specimens, than from any verbal description. In each circlet the plates abut laterally, 

 and alternate with the plates in the circlets above and below, it is strange to find 

 such regularity of plan in so late a representative of the genus. 



§ 316. The plates are marked by raised narrow axial ridges, meeting in sharp 



