ISOMETRA VIVIPARA. 47 



change of shape is produced by the lengthening of the stalk-joints, and it 

 can now be ascertained that there are 18 of them (plate xxui, figure 2). 

 They are still quite short and with the median ring strongly projecting, 

 which gives the stalk a distinctly serrate appearance. It is now only the 

 very youngest joints that are inclosed by the basals. Contemporaneously 

 with the lengthening of the stalk, the oral plates begin to acquire prominent 

 side-edges. In the next stage (plate xxui, figure 3; plate xxn, figure 10) 

 this character of the orals is much more conspicuous, the outturned side- 

 edges being large and thickened; upon the whole the calyx plates are now 

 quite thick, massive, and finely tuberculated. Along the lower edge of the 

 orals a growth-zone is very distinct, indicated by the linear arrangement of 

 the holes, which are very small. The radials have appeared, likewise the 

 anal plate, which is slightly larger than the corresponding radial and thus is 

 formed prior to the latter. The stalk-joints have lengthened considerably, 

 especially the lower ones, and are now more rounded in outline, the median 

 ring being much less prominent; 21 joints are counted. The Pentacrinoid in 

 this stage bears a remarkable likeness to the Cystidean Lepadocrinus quadri- 

 fasciatus, 3 * the likeness being, of course, due only to a superficial analogy. 



In the following stage, represented in plate xxn, figure 11, and plate 

 xxui, figure 4, the radial plates have grown considerably and, like the orals 

 and basals, show a distinct zone of growth. The costals have formed and 

 have assumed the shape of an elongate scale. In the tentacles elongate 

 spicules have developed. The anal plate has already been quite outgrown 

 by the corresponding radial; it covers the corner of the latter and of the 

 adjoining oral. It is of the same dense structure as the other plates. The 

 stalk-joints are about to assume their final shape, but the median ring is 

 still slightly projecting; 24 joints are counted. The terminal stem plate is 

 slightly lobed. The short upper joints are somewhat broader than the rest. 



Some later stages are represented in plate xxui, figure 5; in plate xxn, 

 figure 12, and plate xxni, figure G. In the first of these the rudiments of the 

 two arms have appeared, joining the outer sloping sides of the axillary; the 

 costals still remain very slender. In the specimen shown in plate xxn, 

 figure 12, and plate xxm, figure G, some 6 to 8 arm-joints have developed. 

 The radials are joining one another, thus separating the orals from the basals. 

 The strongly raised side-edges of the orals give them a peculiar shape, some- 

 what like a trough. The corner of the oral plate joining the anal plate has 

 undergone some absorption, and the corresponding radial plate has the 

 adjoining corner truncated. The costals are widening and beginning to assume 

 their final shape. The number of the stalk-joints has not increased. 



The most advanced stage found is that represented in plate xxui, figure 7. 

 Here the first pinnule has made its appearance, on the twelfth arm-joint, 

 which proves that here also the oral pinnules are not the first to develop. 



M See Bather. Echinoderma, in Ray Lankester's "A Treatise on Zoology," HI, 1900, p. 62. 



