1904.' 



Affinities of Palceodiscus and Agelacrinus. 



41 



observation with regard to the flooring plates of Hemicystis. " Deren Zahl 

 nicht immer genau so gross ist wie die Zahl der Saumplattchen, so dass 

 wohl bisweilen je 2 auf einer solchen Subambulacral-Platte aufsitzen." 

 I can find no correspondence in number between the side-covering and 

 the flooring plates. At times, two side covering plates appear to rest 

 on one flooring plate on one side of the groove, figs. 10, 11, but, on the 

 other side, the corresponding plates lie on portions of more than one 

 flooring plate. This may be due, however, to secondary distortion. 



The above account of the structure of the ambulacral groove is 

 confirmed by a study of a specimen of Agelacrinus cincinnatiensis, in 

 which the side-covering plates have been dissolved away along most of 



FIG. 10. View mm. removed or 20 sections -fa mm. thick. 



the ambulacra. The flooring plates, which agree precisely with those 

 A.pileus, have been thus exposed. They show grooves along their 



>rder, which are the articulation of the upper series, Plate 1, fig. 2. 



The flooring plates enclose the mouth, except in the anal inter- 

 radius. In the ground specimen of A. pileus they have been slightly 

 displaced. They are more naturally preserved in the A. cincinnatiensis 

 examined, Plate 1, fig. 2. The fact that the flooring plates do not 

 enclose the mouth in the anal inter-radius is a necessary consequence of 

 enlargement of this inter-radius. The plates of the mouth are merely 

 the flooring plates of the ambulacra. The ambulacra I and V do not 

 meet in the angle of the anal inter-radius, and consequently the mouth 

 ring cannot be completed by these plates. The enlargement of the 



