210 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
Known only from natural casts and impressions from the natural moulds 
in the rock. Calyx large, oblong, its form extremely asymmetrical. Dorsal 
cup deeply depressed interradially, the rays projecting so as to give to the 
calyx a strongly lobed outline. Ventral disk generally as high as the dorsal 
cup; its posterior side inflated from below the brachial zone to the summit, 
forming a conspicuous helmet-shaped protuberance, which at the arm regions 
not only occupies the whole width of the posterior interradius, but encroaches 
largely upon the left posterior ray, not involving the ray to the right, from 
which it is separated by a deep groove. This protuberance, as seen from the 
casts, grows narrower at the summit, and the upper part either bends directly 
upwards, in the form of a central tube, or is continued across the summit to 
the anterior side, where it opens out to the exterior at — or even beneath — 
the arm bases. 
Infrabasals five, elongate, variable in form; four of them, as a rule, 
angular above; the posterior one broadly truncate, supporting the first anal 
plate. Radials decidedly angular below. Costals two; the first generally 
hexagonal. Distichals varying from two to four in different rays, there being 
generally two in the anterior rays against three or four in the posterior ones. 
The interbrachials, which are numerous and of rather large size, pass unin- 
terruptedly from the dorsal to the ventral side of the calyx, and are in 
contact with the interambulacrals. The first plate of the regular sides rests 
deeply between the sloping upper faces of two radials, its lower angle excep- 
tionally touching the basals; there being two plates in the second row, and 
generally three in the third. Anal side wider and larger throughout; the 
first anal plate placed upon the truncated basal, supporting three plates in 
the first row, and three or more in all succeeding rows. The ventral surface 
of the casts is marked by well defined ridges, converging from the arm bases 
to a pentangular, somewhat elevated space behind the anus, the centre of 
which is represented by a small cone. These converging ridges are open 
grooves or galleries at the inner floor of the disk for the reception of subteg- 
minal ambulacra, and the pentangular space in front of the anus represents 
the peristome, of which the mouth occupies the median part. Anal opening 
placed at the distal end of a tube, which is either erect and passes outward at 
the summit of the disk, giving to the mouth an excentric position, or is con- 
tinued beneath the plates of the disk to the anterior side of the calyx, where 
it opens out interradially or interdistichally at — or below — the arm regions. 
Ventral disk covered with comparatively large plates, forming a sort of vault, 
in which neither orals nor covering pieces can be distinguished. 
