III. Lebetodiscm Dickson i. 



1908 547 



governed by a definite arrangement. The diameter of the opening is 

 about 2 mm. ; that of the whole ring is about 4'5 mm. 



The Under Surface is so obscured by a hard matrix that one 

 cannot, even after many days labour, be sure of its structure. 

 Fragments seen below the end of the left posterior ray and of the 

 posterior interradius, towards its right side, suggest that there was 

 a pavement of irregular polygonal plates, about 9 to a square millimetre, 

 not imbricating, but probably set in a flexible integument. A broken 

 edge visible in the left anterior interradius, suggests that the larger 

 plates around the periphery formed a stoutish frame, and that the 

 minutely plated central integument stretched loosely across from this 

 frame. Such a suggestion is at least in accordance with the little 

 that we know of this region in the Edrioasteridae. At the same time 

 there is room for doubt whether all the appearances actually proceed 

 from portions of the individual. 



Remarks on the Holotype and on Billings' Description. 



The type-specimen of this species is " a fragment, consisting of one 

 perfect ray and two of the interradial spaces," preserved in the Museum 

 of the Geological Survey of Canada, at Ottawa. Since this was the 

 only specimen in the possession of the Survey in 1856, I assume it to 

 be the same as that taken by Billings to London, and figured in 

 Decade iii, pi. viii, figs. 3, 3a. These figures, however, show two 

 complete rays and considerable portions of three others. Only two 

 interradial areas, however, are at all complete, and neither contained 

 the anus. No central aperture is to be distinguished. The specimen 

 " is quite flat, and appears to have been firmly attached." There is 

 no reason to doubt that Bigsby's specimen, as well as the "other 

 specimens" alluded to by Billings, were rightly referred by him to 

 this species. 1 Moreover, since his second account was actually based 

 largely on Bigsby's specimen, any differences between that account 

 and the description now given must be differences either of observation 

 or interpretation. Let us consider them. 



"The diameter ... is from three-quarters of an inch to an inch 

 and a half." Billings' fig. 4 represents our specimen as 1-iV inch 

 along a diameter which is really |f inch. It is therefore a medium- 

 sized specimen. The type-specimen, as drawn, would have had 

 a diameter of inch, and is therefore a very small specimen. 



"The rays . . . are bounded by two rows of small plates, which . . . 

 arch over the grooves. The upper ends of the plates on one side meet 

 those of the opposite side, in a line along the centre of the ray, thus 

 forming for each ray a sort of covered way." In other words, the 

 plates called by me ' flooring-plates ' were regarded by Billings as 

 covering-plates, and he did not see the true covering-plates at all, 

 which he was hardly likely to do unless he specially looked for them. 

 But, besides this, the plates in question do not arch over so as to form 

 a covered way, but occupy the full thickness of the ray, as may be 

 seen in section at the end of the anterior ray of Bigsby's specimen. 



1 Should it ever be proved that Bigsby's specimen is of a different species, it will 

 have to receive a new name ; and that new species must then be taken as the genotype 

 of Lebetodiscus . 



