HEXACEINIDiE. 747 



Basal cup large, broadly obconical, its height about three eighths the 

 length of the calyx to the arms, its lower end slightly projecting and trun- 

 cated, the truncated part completely occupied by the large upper stem joint. 

 Eadials gradually spreading, their upper faces nearly one-third wider than 

 the lower, and equal to the length of the plates ; the lower faces in three of 

 the plates a little convex, in the two meeting the interbasal sutures slightly 

 angular; facets deep, semicircular. The outer surface of the radials is 

 marked by a median ridge, rather faint at the lower end, but growing wider 

 and quite prominent upward. Costals two, short, twice as wide as long, 

 rounded on the back. Of the distichals only two single plates are preserved ; 

 they are as wide as, but shorter than, the costals, and are connected laterally. 

 All other parts wanting in the two type specimens, with the exception of 

 a few joints of the stem, which are round and extremely short. The nodal 

 joints are very much the widest, their projecting edges exceedingly thin and 

 knife-like ; while the edges of the internodal joints are rounded. 



Horizon and Localibj. — Lower part of Hamilton group ; Louisville, Ky. 



Type in the collection of the late Major S. S. Lyon. 



ARTHRACANTHA Williams. 



1883, H. S. Williams; Proceed. Amer. Pliilos. Soe. (April), p. 8i. 



1885. W. and Sp. ; Revisioa, Palfeocr., Part III., p. 116 (Proceed. Acad. Nat. Sci. Pliila., p. 338). 



1887. Whiteaves ; Coutr. to Cauad. Palseont., Vol. I., p. 96. 



1889. S. A. Miller; N. Amer. Geol. and Palseont., p. 225. 



Sjn. Hi/siricriam Hiude; 1885, Ann. and Mag. Nat. Hist., p. 158. 



Plates arranged as in Ilexacrinus, but covered with numerous tubercles, 

 each of them having a small pit for the reception of a movable spine. Basals 

 three, large, subequal, and pentangular. Eadials and anal plate as in Hexa- 

 crinus. Costals two, comparatively large, and forming part of the dorsal 

 cup. Distichals three to four, small, curved like arm plates and directed 

 outward ; those of the same ray separated by interaxillaries or in contact 

 laterally. Arms branching, biserial ; pinnules of moderate size. 



Interradial plates numerous, covered like the plates of the dorsal cup, 

 with irregularly arranged, spine-bearing tubercles. Anal opening excentric. 

 Column round. 



Distribution. — Upper Devonian, and only found in the neighborhood of 

 Lake Ontario. 



Type of the genus : A>-tJiracantha itJiacensis Williams. 



