Agelacrinidae and Lepadocystinae 
423 
can Museum of Natural History. In the latter specimen numer- 
ous basal extensions are present. Here it is noted that the exposed 
ovate part of the covering plate forms about half the length of the 
plate. Where it adjoins the margin of the adjacent ambulacral 
plate, the covering plate is bent downward distinctly but not to 
any great extent, and then resumes the same curvature as the 
exposed part of the plate. Immediately beyond the point where 
the covering plate passes beneath the adjacent interambulacral 
plate, the covering plate begins to narrow rapidly, and then the 
sides become approximately parallel, forming the basal extension, 
which only slightly exceeds one-fourth the total length of the plate. 
At the interambulacral angles, where there is little room, these 
basal extensions proceed from the distal side of the covering plates 
and are directed diagonally (plate I, Fig. 6A) toward the median 
parts of the interambulacral area. 
In Agelacrinus holhrooki, No. 1004 of the James Collection in the 
Walker Museum of Chicago University, several of the basal exten- 
sions of the covering plates of one of the rays are sufficiently 
preserved to indicate that the downward flexure of the covering 
plates, at their contact with the adjacent parts of the interamhula- 
cral plates, takes place at about half the length of the covering 
plates, and from, this point the covering plates narrow rapidly, the 
terminal fourth forming that part of the basal extension which has 
approximately parallel sides. The basal extensions seen on 
part of the anterior ray are very narrow (plate I, Fig. IE), while 
those on the left ray (plate I, Fig. IF, are more like those of 
Agelacrinus cincinnatiensis. These differences may be only local 
differences along the length of the ray, or may be due to weathering 
in case of the very narrow basal extensions. Here, again, two 
covering plates appear to occur on each side of the floor plates. 
In Thresherodiscus (plate I, Fig. 8), two lateral covering plates 
also occur on each side of the floor plates. There is no evidence 
whatever of basal extensions of these coverings plates. In fact, 
there is no room for the same. In this respect, the covering 
plates of Thresherodiscus resem.ble those of the Cystidea. 
Basal extensions of the covering plates are unknown also in 
Streptaster (plate I, Fig. 7B). They probably were absent in the 
Devonian and Carboniferous Agelacrinidae, in which the rays 
are very narrow (plate I, Fig. 2). It is possible that the presence 
of basal extensions of the lateral covering plates may prove to be 
