THE STEM AND CALl \. 25 



limited to the Bummit, except in Pentremitidea ledal (PI. V. figs. 12, 13). The 

 same is usually the case in Troostocrinus (PI. V. fig. 2] ; PI. XII. fig. I I ), and also 

 in Tricoelocrinus (PI. XIX. tig. 13), both of winch may have very deeply incised 



radials occupied by long ambulacra, so that the radial limbs extend close up to the 

 mouth. But in Pentremites (PI. II. figs. 1-7), Mesoblastus (PI. VI. figs. 8, L2, 

 13), Granatocrinus (PI. VIII. figs. 14, 1G, 19), Schizoblastus (PI. VIII. fig. 9), Oropho- 

 crinus (PI. XI. fig. 9), and Eteutherocrinus (PI. XIX. figs. 4, G) the radial limbs 

 are more or less roundly truncate and the ventral angles of the interradial areas 

 between the ambulacra are occupied by the deltoid plates, which thus form the walls 

 of the proximal ends of the radial sinuses, just as in Codastcr (PI. XII. figs. 1-6; 

 PI. XVI. figs. 1, 2). The deltoids are small in Mesoblastus and Eleutherocrinus, and 

 the radial limbs therefore are proportionately long ; while no true Pentremite has verj 

 large deltoids, their maximum development being reached in species like P. ovalis, 

 Goldfuss, P. robustus, Lyon, and the P. hemisphericus and P. basilaris of Hambach, 

 so that the greater part of the sinus is bounded by the radial limbs. 



In Granatocrinus and Schizoblastus, however, there is a very great amount of 

 variation in the relative proportions of the radials and deltoids upon the exterior of 

 the calyx. In Schizoblastus melonoides the deltoids are small, so that the radial 

 limbs appear m the ventral aspect at no great distance from the mouth (PI. VI. 

 figs. 15, 16). But in other species of the genus, notably in S. Sayi and S. Eofei 

 (PI. VIII. fig. 10), the radials are greatly diminished in size, and the deltoids form 

 much the greater part of the calyx. Granatocrinus Norwoodi possesses very large 

 radial plates, extending from the edge of the hollow base almost to the very apex of 

 the calyx, and correspondingly small deltoids (PL II. figs. 32-36 ; PI. VII. figs. 3, 5, 

 12). The proportions of the two are nearly equal in G. ellipticus, G. orbicularis, 

 and in some forms of G. campanulatus (PL VIII. figs. 14, 16, 19; PL IX. figs. 11, 

 12) ; while in G. Derbiensis (PL IX. figs. 1-4, 6) the radials are reduced to a minimum, 

 appearing as small pentangular plates, the apices of which are notched to receive 

 the distal euds of the ambulacra, so that the radial sinus is almost entirely bounded 

 by the deltoid plates. 



The radials of Granatot rinus Derbiensis appear in a side view of the calyx ; but in 

 Elceacrinus Verneuili this is only slightly the case, the radials being almost entirely 

 limited to the abactinal surface (PL II. fig. 46). They are notched for the ambulacra, 

 and strongly recall the appearance of the ocular plates in the Urchins. In this genus 

 the radial limbs are only represented by the two sides of the ambulacral notch ; while, 

 on the other hand, they reach a considerable size in many species of Pentremitidea 

 and Triccelocrinus (PL V. figs. 3, 6, 12, 17 ; PL XVI. fig. 18 ; PL XIX. fig. 13). The 

 ambulacra are situated in the deep notch between them, and thus have a nearly 

 vertical position, descending very rapidly outwards from the peristome, which is at 

 the same level as the ventral ends of the radial limbs. In the remarkable genus 



