REPORT ON THE CRINOIDEA. 17 



be a permanent inequality, large and small joints alternating all down, the stem. This is 

 the case, for example, in the fossil Pentacrinus jaccardi and Pentacrinus nicoleti figured 

 by de Loriol,^ and also to some extent in the recent Metacrinus mnseleyi (Pis. XLV., 

 XL VI.). 



There is very little difference between the upper and the under faces of the young- 

 nodal joints, or between either of them and the ordinary internodal joint (PI. XXII. 

 figs. 1, 6-8, 15; PL XXXa. figs. 2, 3; PL XXXVIL figs. 14-16; PL LL figs. 2-5). 

 But as the joints become older and more pentagonal their difterences are more apparent 

 (PL XIIL figs. 2, 3, 5, 6, 10; PL XXIL figs. 16-18; PL XXXVIL figs. 11-13, 17, 18); 

 while in the lower part of the stem the simple syzygial nature of the under face of the 

 nodal joint and its loss of the denticulate petaloid markings become very distinct 

 (PL XV. fig. 6 ; PL XXIL figs. 19, 20 ; PL XXVI. figs. 13, 14 ; PL XXVIL figs. 2, 3 ; 

 PL XXX. figs. 26, 27 ; PL XXXVIL figs. 21, 22 ; PL XXXIX. figs. 4, 5 ; PL XLV. 

 figs. 4, 5 ; PL L. figs. 21, 22). 



In some species of Pentacrinus, e.g., Pentacrinus ivi/viUe-thomsoni, the lowest and 

 therefore the oldest stem-joints gradually lose the more or less prominent ridges which 

 appear on the faces of those higher up the stem, and become much more smooth and 

 simple in their character (PL XXIL figs. 23-26). 



A similar change seems to take place in Pentacrinus asteria (PL XIIL figs. 10, 11), 

 and also in other species, though I have not been able to trace it so distinctly as in 

 Pentacrinus ivyville-thomsoni. It is manifested externally by the gradual disappearance 

 of the crenulation of the interarticular lines, which is so very prominent in the upper and 

 midcUe parts of the stem (PL XIIL figs. 7, 8 ; PL XV. figs. 1, 2, 4 ; PL XIX. figs. 2-5 ; 

 PL XXV. ; PL XXVIL fig. 1 ; PL XXXI. fig. 8 ; Pis. XXXV. -XXXVIL ; PL XLI. 

 figs. 1, 5, 15; PL XLIILfig. 1). 



The amount of crenulation varies considerably in diff"erent species, according to the 

 position of the large teeth bordering the' outer ends of the petaloid spaces. When these 

 start from near the edge of the joint, as in Metacrinus cingulatus, Metacrinus nohilis, or 

 Metacrinus costatus (PL XLI. figs. 1-3, 5-7 ; PL XLIX. figs. 3-5), the interarticular line 

 is well crenulated. But there is sometimes a sort of rim outside the ends of the teeth, as 

 in Metacrinus murrayi and Metacrinus varians (PL XLI. figs. 15, 17 ; PL XLVII. figs. 

 6-9), and the external crenulation is then less marked. This outer rim is only formed 

 comparatively late, the teeth of a young joint starting directly from its edge, as is well 

 shown in Pis. XXIL, XXIIL, and XXXVIL 



The increase in the length of the internodes only takes place gradually, and i^ciri 

 passu with the continual formation of new joints just below the calyx. Hence, in the 

 upper part of the stem, there is a variable number of premature internodes, those nearest 

 the summit being the shortest, and consisting of the smallest number of joints. The 



1 Swiss Crinoids, pp. 130, 140, pL xv. figs. 13, 36. 

 (ZOOL. CHALL. EXP. — PART XXXTI. — 1884.) li 3 



