TROPIOMETRA CARINATA. 21 



On some of the joints small separate pieces are seen apparently soldered to 

 the main piece of the joint, and I am inclined to think that this is the destiny 

 of all these small separate calcareous pieces, even the fairly large one lying 

 between the sixth and seventh joints, counting from the terminal plate. 

 Seeliger (pages 229 and 324) finds such intercalation to occur in Antedon, 

 but only between the second to third upper (younger) joints, and only where 

 the distance between the joints is larger than usual. This evidently means 

 that there is only a shifting in the time for the first appearance of the joints, 

 as is actually the case with the first joint in plate ix, figure 1. 



In plate ix, figure 4, is figured an embryo 40 hours old, seen from the 

 ventral side, the stalk lying thus directly under the vestibulary imagination. 

 The skeletal plates have grown considerably, forming large fenestrated plates. 

 A comparison with plate ix, figure 3, shows one of the basal plates apparently 

 in quite a different position from what is seen in that figure, reaching nearly 

 to the terminal stem-plate. The difference is, however, only apparent; on 

 turning figure 3 to the same position as figure 4, the basals will occupy a 

 corresponding position, and what change has occurred is due mainly to the 

 increase in size. The infrabasals are fairly large, fenestrated plates, hidden 

 by the overlying basal plates, but quite distinctly seen on close observation. 

 They do not appear to be of different size. The number of stalk-joints 

 appears to be 10. 



Plate x, figure 3, represents a normal embryo shortly after the fixation. 

 The oral and basal plates have assumed their final position, forming the 

 calyx, and the stalk-joints have begun to grow in length. There are 10 of 

 them, the upper one mostly hidden by the basal plates. The infrabasalia can 

 not now be seen, on account of the opaqueness of the embryo. Figures 4 

 and 5, plate x, show only growth-changes, depending mainly on the pro- 

 longation of the stalk-joints; new joints have not appeared. The vestibulum 

 has not yet opened; there is only a depression, showing the place of the 

 future opening. 



The fully formed young Pentacrinoid is shown in plate x, figure 6. The 

 oral valves have separated and the tentacles are protruding. No new stalk- 

 joints have appeared as yet. The terminal stem-plate has developed into a 

 fairly large, irregular, roundish disk consisting of a thick, reticulate network. 

 The oral and basal plates begin to form reticulations. It is a conspicuous 

 feature that the upper edge of the basal plates embraces the lower edge of 

 the oral plates. 



The oldest stage reached is that seen in plate x, figures 7 and 8. The 

 anal plate has been formed, but there is as yet no trace of the radials. The 

 basalia are still seen to have at their upper edge a wider circumference than 

 that formed by the oralia, so there is a characteristic offset between the two 

 circles of calyx plates, a feature which does not appear to be due to the pres- 

 ervation. The plates are already considerably thickened, strongly reticu- 



