16 CATALOGUE OF THE BLASTOIDEA. 



such as the Tertiary R. Thorenti and the recent R. Raivsoni ; and there is a further 

 similarity between the two types, for the long basal cup is not unfrequently 

 marked by one or more circular constrictions, which are sometimes so distinct as to 

 have been taken for sutures both in the Blastoid and in Rhizocrinus (PI. V. figs. 25, 26). 



The basals of Phaenoschisma resemble those of Pentremitidea (PL V.), being lowest 

 in P. caryopliyllatum (PI. XIV. figs. 1, 4), and highest in P. Archiaci (PI. XII. 

 fig. 10). Those of Orophocrinus (PL XIV. fig. 17 ; PI. XV. fig. 1 ; PL XVI. figs. 5, 

 7, 8) are also relatively high, except perhaps in 0. Puzos (PL XIV. fig. 14). In all 

 these four generic types, as also in Pentremites piriformis (PL II. figs. 24-30) and 

 its allied forms, the basal cup expands uniformly upwards from its lower to its upper 

 edge. This is also true of some species of Codaster as C. alternatus, C. pyramidatus, 

 and C. Ilindei (PL XII. figs. 2, 3, 5). But in the British species (PL XIII. 

 figs. 10, 12, 15) this is not the case. For a short distance above the articular facet 

 the basal cup expands but little ; but it soon opens out somewhat rapidly until it 

 reaches the full diameter of the calyx, or nearly so. 



In all these types, with the partial exception of Codaster trilobatus (PL XIII. 

 figs. 2, 13), the general outline of the basal cup is that of a tolerably regular 

 pentagon (PL IV. fig. 16 ; PL V. figs. 8, 11, 14, 29). But in Troostocrinus, Tri- 

 ccelocrinus (PL XIX. fig. 14), and Stephanocrinus it is very distinctly triangular. 

 The radials form a regular pentagon, the sides of which are interradial. But the 

 side representing interradius A-B is not continued down on to the basal part of the 

 cup ; for the two ridges which bound it approach one another below and unite into 

 the single ridge which marks the middle line of the azygos basal x (PL XIX. fig. 14). 

 The next interradius B-C forms one side of the basal triangle ; and another is 

 compounded of the two interradii C-D and D-E, owing to the gradual obliteration of 

 the ridge which separates their upper ends, while the third side of the triangle 

 corresponds to the interradius E-A. The three angles of the trihedral base thus 

 coincide with the median lines of the three basals, while its sides correspond to the 

 interbasal sutures (PL XIX. fig. 14). In Troostocrinus and in Stephanocrinus the 

 base narrows down to a triangular lower surface, in the centre of which is the 

 circular facet for articulation with the stem. But in Tricoslocrinus, which has a 

 relatively wider cup supported by lower and more spreading basals, the lines of the 

 three sutures which correspond to the flat sides of the Troostocrinus base, are " so 

 very profoundly and broadly excavated as to impart a very remarkable appearance to 

 the lower part of the fossil." 1 A slight indication of this character also appears in 

 some specimens of Codaster trilobatus (PL XIII. fig. 2). 



The calyx of many Blastoids is marked by very well defined ridges which start 

 from the bottom of the basal cup and extend upwards until they meet the radial lips 



1 Meek and Worthen, licport Geol. Survey Uliuois, 1S73, vol. v. p. 507. 



