HEXACRINIB^. 751 



angular along the median line. Costals two; the first narrow and very 

 short; the second sharply angular above; its sloping njjper 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 ai^parently 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 uj^per end of 

 short joints with sharp, knife-like edges. 



Horizon and Xoca%. — Hamilton group; Arcona and Bartlett's Mill, 

 Ontario, Canada. 



Remarks. — Dr. G. J. Hinde described this species as Hi/siricrimis Carpen- 

 teri. We have heretofore, in Part III. 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. S. 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. Eadials 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 



