1908 550 F. A. Bather — Studies in Edrioasteroidea. 



clear that the side-plates, here called flooring-plates, are homologous 

 with the flooring-plates of Edrioaster. Whether those plates have 

 homologues in the Agelacrinidae is matter for debate ; at any 

 rate, no genus of that family has similar plates with intervening 

 depressions so like pores. The covering-plates also seem homologous 

 with the covering-plates of Edrioaster, and it is doubtful whether the 

 so-called ' ambulacrals ' of the Agelacrinidae are of the same nature ; 

 if they are, they have, at any rate, different relations to the adjoining 

 plates. 



Proceeding then to compare the species with other Edrioasteridae, 

 we note that it differs from them in the restriction of the grooves to 

 the actinal surface, in the small size of the covering-plates, and in the 

 absence or very slight development of an abactinal frame. 



The species therefore appears to represent a generic type hitherto 

 unrecognised, and a type of considerable interest in that it is inter- 

 mediate in so many features between Edrioasteridse and Agelacrinidae. 

 What bearing this may have on the classification of the Edrioasteroidea 

 must be reserved for discussion after more of these Studies shall have 

 been published. For the present, the following diagnosis may be 

 offered. 



Lebetodiscus 1 gen. no v. 



An Edrioasteroid, with theca flattened below, convex above ; no 

 marginal zone on actinal surface ; no definite abactinal frame ; inter- 

 radial thecal plates relatively large, with slight adoral imbrication ; 

 rays curved [contra-solar in genotype], and reaching but not passing 

 the periphery ; subvective skeleton of alternating flooring-plates, with 

 intervening pores, and small irregular covering-plates. 



Genotype : Lebetodiscus Bichsoni (E. Billings, sub Agelacr mites)? 

 Lower Trenton Limestone, Ottawa. 



It may be worth noting that a restored representation of the fossil 

 herein described, by J. S. and A. B. Wyon, adorns the reverse of the 

 Medal founded in 1887 by Dr. Bigsby and awarded biennially by 

 the Council of the Geological Society of London. To judge from the 

 illustration facing p. 252 of Mr. Horace Woodward's " History " of the 

 Society (1907), the medal gives a good general idea of Lebetodiscus 

 Bichsoni. 



EXPLANATION OF PLATE XXY. 



The Upper Figure is taken from a photograph of Big-shy's specimen of 

 Agelacrmites Bichsoni Billings, the type of Lebetodiscus, enlarged two diameters. 

 Only a part of the matrix is shown. The posterior interradius, with the anus, 

 is towards the observer. 



The Lower Figure represents the Lower Trenton Limestone of Table Hock at 

 Chaudiere Falls, Ottawa River, where Bigsby collected the specimen figured above. 



1 Lebetodiscus, from Ae'jSvjs, a cauldron ; after the Chaudiere Falls ; and SiVkoj, 

 a round plate. 



2 But see footnote 1, ante, p. 547. 



