REPORT OF THE DIRECTOR TQTO 



223 



canals at the suture to form pelvislike depressions that com- 

 municate by a vertical canal with the interior. If Palaeocrinus, 

 with practically only three circlets of plates of five each, needed 

 such an elaborate system of stroma strands or muscular processes 

 to hold its plates together, why do we not hnd a similar arrange- 

 ment in Cliocrinus with its hundreds of separate plates? These 

 structures are canals and each was occupied by a branchial 

 vesicle. 



Plate development and the serial formation of branchial 

 vesicles. An examination of plate 5 will show that the epithecal 

 extensions of the b.vs. become shorter and less prominent as we 

 pass from the middle of sutures toward their ends and figure i 

 of plate 6 will show that the shortening is correlated with a 

 decrease in diameter of the sutural canals. On this evidence we 

 should be warranted in assuming that these structures were 

 serially formed and that those 

 nearest the ends of the sutures 

 were youngest. 



As the lateral extension of a 

 plate takes place only by the 

 addition of stereom to its mar- 

 gins we may, without great 

 error, outline a plate as it 

 existed in some earlier stages of 

 its development. This we have 

 essayed to do in text figure 9. 

 It will be at once seen that the 

 smaller the size we make the 

 plate the fewer b.vs. it could 

 have possessed and the inner 

 outline shows a condition where 

 there was but a single b.v. to a 

 suture. These are at the ends 

 of those longest epithecal canals 

 which we have already deter- 

 mined to be the oldest and the Fig. 9 view of a portion of the holotype of 

 , . Palaeocrinus striatus Billings. From a 



snorter ones simply did not photomicrograph, xio. Outlines show prob- 

 , . . able forms of the plates at earlier stages in 



exist at this time. their development. 



We may express a law concerning these b.vs. as follows. On 

 any suture the longest b.v. is the oldest and the newer members 

 were added serially on one or both sides of this as the suture was 



