HEXACRINIDZ. Tol 
angular along the median line. Costals two; the first narrow and very 
short; the second sharply angular above; its sloping upper faces concave. 
Distichals three in the calyx; those of the same ray in contact laterally, or 
' separated by an interdistichal. Arms biserial, generally from the first free 
“plate. There are three interbrachials to each regular interradius, of which 
the inner one is larger than the two outer, the latter curving outward so 
as to meet the distichals. ‘The interbrachials are followed by numerous rows 
of small, rigid interambulacral pieces, of which the upper ones from each 
side meet in the summit, there being apparently no orals. The first anal 
plate is narrower than the radials; it is succeeded by five plates, of which 
the middle one is larger and somewhat bulging. Anus excentric, placed 
within a small protuberance. Column round, formed at its upper end of 
short joints with sharp, knife-like edges. 
Horizon and Locality.— Hamilton group; Arcona and Bartlett’s Mull, 
Ontario, Canada. | . 
Remarks, — Dr. G. J. Hinde described this species as Hystricrinus Carpen- 
teri. We have heretofore, in Part II]. of the Revision, stated that in our 
opinion Williams’ prior name should be retained. Williams’ description, 
although merely comparative, is amply sufficient for the identification of the 
species; this view is also held by Professor Whiteaves and Mr. 8. A. Miller. 
Arthracantha depressa W. and Sp. (nov. spec.). 
Plate LXXVI. Figs. 3a, b. 
Dorsal cup short, twice as wide as high. Basals so closely anchylosed 
that the lines of union are rarely seen; they form a very shallow, hexagonal 
basin, which near its outer margin is surrounded by two indistinct corrugated 
ridges. A single ridge, even more obscure, follows the lower margins of the 
radials, parallel to the basi-radial sutures. Radials once and a half as wide as 
long, their lower faces nearly straight, the upper ones to fully one half their 
width deeply excavated, their outer ends truncated and distinctly sloping. 
Costals two, on the same plane with the radials, unusually large for the 
genus, both of them constituting a part of the dorsal cup; they are three 
times as wide as long, and of a similar form, except that the first is angular 
below, the second angular above. Of the distichals only the two lower ones 
take part in the calyx; they are quadrilateral, and twice as wide as the 
succeeding ones. The free distichals are cuneate to the second or third 
I LY a 
_ 
panacea 
