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 outhne. 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 tw^o 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 w^hich the mouth occupies the median part. Anal opening 

 placed at the distal end of a tube, which is either erect and passes outw^ard 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. 



