414 
Aug. F. Foerste 
ing plates are at least as numerous as the lateral covering 
plates, in some parts of the specimen apparently exceeding the 
latter in number (plate I, Fig. 8). It is the median covering 
plates which, in this species, interlock along the median line of 
the rays, since the lateral covering plates do not reach this line. 
It may be noted that even in Agelacrinus cincinnatiensis a third 
series of still smaller plates may be intercalated on each side of the 
median line, occasionally, producing a very serrate appearance 
along this line. 
In Cystasier granulatus each ray has about eleven pairs of 
covering plates. As in Agelacrinus pileus, the spinous prolon- 
gation of each covering plate is on the proximal side of the tip. 
The covering plates are flattened, and the upper parts of the 
flattened surfaces are inclined distally, or away from the center 
of the specimen. 
17. Peristomial Plates 
The peristomial plates of Thresherodiscus do not appear to 
differ conspicuously from the other covering plates. They are 
not well preserved, but such traces as exist suggest merely a con- 
tinuation of the series of median and lateral covering plates of the 
rays also along the peristomial slit. The term peristomial slit is 
used here for the median line between the covering plates of the 
ambulacral system along that part which extends across the oral 
aperture, from the point of bifurcation of rays 1 and 2 (left pos- 
terior and left rays) to the point of bifurcation of rays 4 and 5, 
(right and right posterior rays) . In this lack of differentiation of 
the covering plates in the peristomial region from those on the 
rays, Thresherodiscus resembles the more primitive conditions in 
the Cystidea. 
The peristomial plates of Agelacrinus pileus (plate I, Fig. 5B, 
also plate II, Figs. 1, 2) are highly differentiated from those of the 
rays. In all specimens there is a right anterior (R) and left 
anterior (L) peristomial plate. These are rhomboid plates, 
sufficiently extended along their greater diameters to prevent the 
base of the anterior ray from coming in contact with the base of 
the right and left rays. Of these, the right anterior rhomboid 
plate always is taller, so that the first covering plate of the anterior 
ray is found on the left side of the median line. On the posterior 
side of the peristomial slit there is a single large plate (P), often 
