FAUNA OF THE TRENTON GROUP. 
5 
differences may be due to tbe imperfection of tbe specimen studied, 
and as regards tbe third difference, the “ flooring plates ” of Bather 
ini Lebetodiscus are presumably the same as the “ outer covering 
plates” of F oerste, and Bather’s specimen was not so preserved as 
to enable him to get at the real flooring plates, which in a Canadian 
specimen are concave and single, not double. 
(Compare Dr. Bather’s fig. 1, p. 545, with Dr. Foerste’s fig. 1, 
PI. 1, fig. 4, PI. 2, and fig. 4, PI. 3, or, for the genus Thresherodiscus, 
fig. 8, PL l). 1 The small plates which Dr. Bather took for the real 
covering-plates are the {t median or intercalated covering plates ’’ 
of Foerste. No real difference is apparent between the structure of 
the subvective system of Lebetodiscus and such a typical (Ordo- 
vician) Agelacrinites as A. pihus, except in the large pores between 
the lateral covering plates. Foerste 2 calls attention to the fact that 
in Lebetodiscus dicksoni all the rays are contra-solar; that the 
supra-oral plates differ from the lateral covering plates of the arms 
merely in their smaller size; that from the median ridge of the 
covering-plates short ridges extend off laterally, except at the tip 
of the plates, where the median ridge broadens out; and that there 
are no accessory plates along the median line of the rays. This 
combination of characteristics he seems to consider of generic 
importance. 
Lebetodiscus dicksoni (Billings). 
Plate I; Plate III, figure 1, 
Billings, Kept, of Prog., Geol. Surv., Can., 1857, p. 29-4; Oan. 
Org. Bern., dec. 3, 1858, p. 84, PI. 8, fig, 3, 3a, 4, 4a; Chapman, 
Expos. Min. Geol. Can., 1864, p. 11(>; Grant, Trans. Ottawa Field 
Nat. Club, vol. 1, No. 2, 1881, fig. 9; Jadkel, Stamm. Pelmat. 1889, 
p. 50, pi. 2, fig. 2; Clarke, Bull. N.Y. State Mus. 49, 1901, p. 191, fig. 
3; figured without name by Sowerby, Zool. .Tour., 1825, 2, p. 318, PI. 
11, fig. 5. 
The Victoria Museum contains four specimens of this rare 
species; the type; a specimen collected by Billings (No. 1415); a 
specimen collected by Mr. Fitzpatrick at Peterborough, Ont. (No. 
1412); and the specimen from Ottawa donated by Sir James Grant 
and figured by him in 1881 (No. 437). 
The type-specimen is very poorly preserved. Billings’ speci- 
men, also poorly preserved, has been cut so as to expose a section 
across arms II and' III, the section of the anterior arm showing 
that the structure is the same as in Agelacrinites pileus, there being 
1 Bull. Sc. Bab., Denison TJniv., vol, 17, 1914. 
2 Bull. Sc. Dab., Denison TJniv., vol. 18, 1917, p. 341. 
