82 STUDIES IN THE DEVELOPMENT OP CRINOIDS. 



not be the case were the joints beyond the second of a totally different 

 morphological value. Also in Tropiometra carinata I have observed a pin- 

 nule branching, not at the second joint, as should be required according to 

 Clark's view, but much farther out, at what joint can not be ascertained 

 definitely, as the pinnule was found broken off, the basal joints lacking; 

 still, 3 joints are left below the axillary, so that the branching occurs here not 

 lower than the sixth or seventh joint. 



The formation of the stalk-joints was found to be in accordance with the 

 previous observations by W. B. Carpenter, Bury, and Seeliger, the joints ap- 

 pearing at first as a half-moon-shaped spicule, the concave side being directed 

 ventrally; new joints are formed only at the proximal end of the stalk, 

 the last of them developing into the centrodorsal, either in connection with 

 the infrabasals or, in cases where the infrabasals are lacking, by itself alone. 



Seeliger (op. cit., pages 229, 324) finds that new joints may be intercalated 

 between the previously formed ones, concluding from the smaller size of 

 such joints. Similar observations were made on Tropiometra (plate ix, 

 figure 1). However, this hardly means a true intercalation, but only a small 

 delay in the appearance of such joints, the place being left open for them (see 

 pages 20-21). The small grains seen between the stalk-joints in plate ix, 

 figure 3, might look more like true intercalated young joints; still their very 

 varying position suggests that they are not destined to form separate joints, 

 but to be soldered with the adjoining columnals. Otherwise this is evidently 

 only an abnormal case. 



The absence of the terminal stem-plate in Thaumatometra nutrix and 

 the presence of supplementary terminal plates in Notocrinus can hardly 

 have any bearing on the morphological meaning of this plate, both these 

 interesting facts being evidently due to the special life conditions of the 

 Pentacrinoids in these two species. 



The homology between the stalk and the centrodorsal plate of the Cri- 

 noids, or especially the central plate of Marsupites and Uintacrinus, and the 

 suranal plate of Echinoids maintained by A. H. Clark, I can not accept, 

 even as a "potential" homology. Upon the whole, I think the homologies 

 supposed to exist between the apical plates of Echinoids and the calycinal 

 plates of Crinoids rest on mistaken conceptions. But a discussion of these 

 problems may be left for a future occasion. 



