Agelacrinidae and Lepadocystinae 
433 
long, large column, and occasional parts of the basal portion of the 
cup. The southern exposures on Cloche Island evidently are of 
Black River age, and therefore these most northern exposures on 
Goat Island probably belong low in the Trenton section. They 
are correlated provisionally with the Kirkfield and Hull horizons 
of Ontario and the Curdsville bed in central Kentucky. The fol- 
lowing is a description of the Thresherodiscus ramosa, here found. 
Genotype, Thkesherodiscus ramosa 
The generic name is given in honor of Mr. and Mrs. J. B. 
Thresher, of Dayton, Ohio, in appreciation of the many years 
of encouragement given the writer in his efforts at scientific in- 
vestigation. The type forms No. 8446 in the collections of the 
Canadian Geological Survey, in the Victoria Memorial Museum 
at Ottawa, Canada. 
Theca discoid, 16 mm. in diameter. Upper surface gently 
convex, excepting at the sides where the slope becomes steep. 
Vertical height between 4 and 5 mm. Lower surface not ex- 
posed, apparently resting upon the surface of some bryozoan. 
Animal probably not permanently sessile, but capable of chang- 
ing its location. 
Mouth central. From this mouth three ambulacral rays di- 
verge — an anterior, a right, and a left primary ray. The right 
and left primary rays form an angle of 150 degrees with each 
other; the anterior ray is directed sufficiently far to the left to 
make an angle of about 90 degrees with the left primary ray. 
All these primary rays bifurcate dichotomously at least twice, 
the first of these bifurcations taking place about 2 mm. from 
the center of the oral area. 
Regarding the trimerous radial structure as primitive among 
the Echinoderma, and the more obvious pentamerism of this, 
group as secondary, the correlation of the ambulacral rays of 
Thresherodiscus (Fig. 1) with those of other Agelacrinidae 
becomes obvious. Designating the left posterior ray of the ordi- 
nary Agelacrinidae as No. 1, and the other rays in succession 
in dextral order as 2, 3, 4, and 5, the last number is given to the 
right posterior ray. Using the same numbers for the rays of 
Thresherodiscus, the anterior primary ray (A) evidently corre- 
sponds to No. 3. The first branches of the right primary ray 
