422 
Aug. F. Foerste 
22. Basal Extensions of Lateral Covering Plates 
OF THE AmBULACRAL RaYS 
The specimen of Agelacrinus pileus described by Miller and 
Faber exposes in a remarkably clear manner the basal extensions 
of the lateral covering plates of the ambulacral rays (plate I, 
Figs. 5A, B, C, D; also plate II, Fig. 4). These extensions 
project laterally from the bases of the covering plates, where they 
rest upon the floor plates, toward and beneath the adjacent 
interambulacral plates. They are best exposed along the anterior, 
right, and right posterior rays, where they are seen for almost the 
entire length of the ray, and on both sides of the ray, but some of the 
basal extensions are seen also in case of the left and left posterior 
rays. Along that part of the base of each covering plate which 
serves as its fulcrum, there are two striations (plate I, Fig. 5C), 
which extend in a direction parallel with the ray, and a moderate 
distance apart. These striations seem to fit against the inner 
margin of the narrow lateral groove which is seen on the upper side 
of the floor plates in certain species. From these bases, of the 
covering plates, the short basal extensions project outward and 
downward at rather a strong angle with the major part of the 
covering plate. The basal extensions are wider on the convex side 
of the rays, and narrower on the concave side, where there is less 
room. On the convex side of the rays the basal extensions have 
a length of about 0.5 mm., and narrow from a width of two-thirds 
of a millimeter, parallel to the length of the ray, to a width of 
two-fifths of a millimeter at the truncated tip of the extension. 
On the concave side of the ray, the basal extensions are much 
narrower, and terminate more acutely; here they have a length 
of about 0.4 mm. At the angle of junction of the rays, where also 
the room is restricted, even the basal extensions on the convex side 
of the rays are narrow and acute. Apparently two lateral plates 
occur on each side of each floor plate, in the case of the few floor 
plates preserved. 
In some specimens of Agelacrinus cincinnatiensis, the basal 
extensions of the lateral covering plates are very well exposed. 
They are shown by specimen 1008-c (plate I, Fig. 6B), in the 
James Collection, at Chicago University, and also are seen along 
both sides of the left ray, and on parts of the anterior and left 
posterior rays of specimen No. 13266-Uc, belonging to the Ameri- 
