I'm: STEW \\l> CALYX. 29 



interradial plates of the Pelmatozoa haw yel to be worked oul ; while the relation 

 of the interradiala to the hydrospires of a Blastoid renders them additionally inter- 

 esting, both to the morphologist and to the systematise They are of very variable 

 size, sometimes as in Schizoblastm Eofei (PI. VIII. fig. 9), Granatocrinus Derbienns 

 (PI. IX. figs. 1, 2, 6), and Elasacrinus (PI. II. figs. 43, 46), constituting almosl the 

 whole of the calyx; and sometimes, as in Cryptoschisma (PI. V. fig. 24 ; PI. XIII. 

 fig. 20), Phmoschisma (PI. XIV. fig. 11), and Tricaslocrinus (PI. XIX. fig. 13), being 

 entirely confined to the summit and almost invisible externally. Their proximal 

 ends form the boundaries of the central mouth-opening, the angles of which are 

 extended in the direction of the rays into five slits between their contiguous lateral 

 edges (PI. I. figs. 5, 6). A little further from the centre these edges meet one 

 another, and form a suture which is naturally covered up by the ambulacral struc- 

 tures, and only becomes visible after they have been removed (PI. I. rigs, (i, 7; 

 PI. XII. fig. 13; PI. XIV. fig. 12; PI. XV. figs. 4, 10); or the summit ground 

 away a little (PI. XL fig. 7 ; PI. XII. fig. 9) ; or in an internal cast of the calvx 

 (PI. VI. figs. 18-20; PI. VII. figs. 7-9). 



Their general outline is either triangular, rhomboidal, or pentagonal. A good 

 instance of the first type occurs in Cryptoblastus inch (PL VII. figs. 14, 15), the 

 interradial sutures meeting the base of the triangle ; while almost the same thing 

 occurs in Schizoblastm Bofci (PI. VIII. fig. 9), and in some forms of Granatocrinus 

 Derbiensis (PI. IX. fig. 6) which have a nearly horizontal radio-deltoid suture. In 

 other types, however, such as Grematocrinus ellipticus (PI. VIII. figs. 16, 19), Codaster 

 (PI. XIII. figs. 1, 4, 8), Orophocrinus (PI. XI. fig. 9 ; PI. XV. fig. 6), and in most 

 Pentremites (PI. II. figs. 1-9, 16-30) the radio-deltoid suture is strongly angular, 

 so that the general outline of the interradial, as it appears exte.mally, becomes 

 rhomboidal, or even pentagonal if its proximal end is truncate, as sometimes occurs 

 in Granatocrinus Norwoodi (PI. VII. figs. 3, 10, 12). It must always be remembered, 

 however, that the real lateral edges of the deltoid plates are concealed beneath the 

 proximal ends of the ambulacra, and only become visible when the lancet-plate and 

 its coverings have been removed (PI. I. figs. 6, 7 ; PI. XII. fig. 13 ; PI. XV. figs. 4, 10). 

 Internal casts, such as those of Granatocrinus Norwoodi figured on PI. VII., are of 

 special value in elucidating this point, the sutural edges of all the plates being 

 indicated by more or less well-defined ridges on the surface of the cast. 



Except for the presence of the spiracles round the inouth, the deltoids of the 

 Blastoids have the same general relation to the hydrospires as the radials have. In 

 Granatocrinus Derbiensis (PI. IX. figs. 1, 2, 6), Schizdblastus Eofei (PI. VIII. fig. 10), 

 and in Elceacrinw (PI. II. figs. 43-46) the sides of the long and narrow ->inus are 

 almost entirely formed by the deltoids, the radials being relatively very small, and 

 only supporting the distal ends of the hydrospires. On the other hand, if the radials 

 are long, as in Granatocrinus Norwoodi (PI. II. figs. 32-36), Pentremitidea ledal 



