MELOCRINID. 307 
Distribution. — Allocrinus is restricted to America. It belongs to a little 
group of Crinoids which is sparingly represented in this country, but of which 
several genera with a number of species are known to have existed during 
the Upper Silurian period in Sweden and England. 
Remarks.-— This genus has close affinities with Patelliocrinus and Dolato- 
crimus. It differs, however, from both of them in the form and size of the 
various calyx plates, and in having uniserial arms. It also approaches Cen- 
trocrinus, but this has a large and perfectly anchylosed basal disk. 
Allocrinus typus W. and 8p. 
Plate XXIV. Figs. 7a, b. 
1889. W. and Sp.; Geol. Rep. Illinois, Vol. VIII., p. 207, Plate XIV., Fig. 7. 
Below medium size. Calyx depressed, proportionally small, sharply 
pentangular in outline, the arm bases projecting, the arms massive and 
proportionally stouter than those of any other Crinoid; the plates with- 
out ornamentation. Basals small, forming a flattened disk, which is almost 
covered by the column; interbasal and basi-radial sutures indistinct. Radials 
large, abruptly curved, the lower portions forming the rim of an inverted 
shallow basin, the upper half bent abruptly upward and forming together 
with the plates above a shallow, spreading cup with straight sides. Costals 
and fixed distichals short, curved like arm plates, producing upon the sur- 
face of the calyx strong, rounded ridges, which insensibly pass into the free 
arms. Arms ten, stout, uniserial, free above the second plate; composed of 
transverse joints with parallel upper and lower faces, The arms much wider 
midway than at either end, tapering considerably and uniformly toward the 
tips, but almost as much toward the calyx. Interradial spaces deeply de- 
pressed, composed of two plates vertically arranged, of which the first is the 
larger and nodose, and extends to the top of the costals ; while the other is 
on a level with the fixed brachials, and forms, to some extent, a part of the 
ventral surface. Nothing is known of the anal aperture and the structure 
of the ventral disk. Column round, small; axial canal pentangular. 
Horizon and Locality. — Niagara group; Wayne and Decatur Cos., Tenn, 
Types in the Worthen collection, Springfield, and that of Wachsmuth and 
Springer. 
