1904.] Affinities of Palceodiscus and Agelacrinus. 43 



The mouth ring is further strengthened by plates, which underly 

 the mouth-covering plates. These occupy the angles between II and 

 III and III and IV. The third, which underlies the large mouth- 

 covering plate of the anal inter-radius, completes the mouth ring in 

 this region. It has been somewhat attacked by solution in both cases 

 examined. The flooring plates of the mouth ring are thicker than 

 those of the remainder of the ambulacra. No trace of any plates 

 are to be observed underneath them. This would seem to indicate 

 that the central part of the ventral region of Agelacrinus was occupied 

 by a membrane with calcification weakly, or not at all developed. A 

 similar appearance has been described in Edrioaster by Forbes and 

 Bather. Bather has supposed that by the pulling in of this membrane 

 a vacuum would be produced, thus fixing the animal to the sea floor. 



No trace of any Madreporite was observed in Agelacrinus. Jaekel 

 demands the absence of this organ throughout the Edrioasteroidea. 

 Bather has, on the contrary, interpreted a structure on the thecal plate 

 near the mouth in the anal inter-radius of Edrioaster as a Madreporite. 



Theoretical Conclusions. We have seen that there are two distinct 

 views as to the structure of the ambulacrum in the Edrioasteroidea. 



1. That which has found its latest supporter in Jaekel asserts that 

 the ambulacral groove possesses single flooring plates. No openings 

 are present between these, or, if they were present, they served as 

 genital orifices. 



2. That which is advocated by Dr. Bather, founded upon a study of 

 Edrioaster. This states that there was a double series of alternate flooring 

 plates. Between these were openings, through which passed ampullae. 



The results of the above investigation shows that Jaekel is correct 

 with respect to the structure of the ambulacral groove in the 

 Agelacrinidse. There appears no reasonable doubt, however, that 

 Edrioaster possesses a double series of alternating flooring plates, and 

 between these plates are pores. Dr. Bather has been kind enough 

 to show me an exceptionally well-preserved specimen of Edrioaster 

 Bigsbyi, which leaves no doubt upon this point. Dr. Bather's view 

 that through these pores protruded either podia or ampullae, is, 

 however, open to criticism. The pores are at the extremity of a 

 groove, in such a position that they lie immediately under the hinge 

 of the covering plates. The groove which leads to the pore opens out 

 gradually, and suggests that it served to carry water to the pore, 

 which would then have a respiratory function. That some members of 

 the Edrioasteroidea should show respiratory pores, whilst others do 

 not, is in accordance with the structure of the group as a whole. 

 Jaekel divides the Edrioasteroidea into two sub-groups. The The- 

 cocystideae, including Edrioaster ; and the Agelacrinidae. The former 

 are characterised by their firm skeletons, the latter by their imbri- 

 cating plates. The forms with firm skeletons would require respiratory 



