Agelacrinidae and Lepadocystinae 
413 
In Agelacrinus cincinnatiensis , and in the closely related Agela- 
crinus holbrooki (plate I, Fig. 1C), four series of cover plates are 
present, two series on each side of the median line of the ray. Of 
these series, the two outer ones are here called the lateral coverings 
plates, and the two inner series, intercalated along the median line 
are called the median or intercalated covering plates. The 
exposed part of the lateral covering plates are triangular in form, 
the apices being directed toward the median line ; they are of larger 
size and rest upon the lateral margins of the floor plates. The 
median or intercalated covering plates are of smaller size and only 
their tips may be seen, intercalated between the tips of the lateral 
covering plates, and about equalling the latter in number. Along 
the ambulacral groove these plates are probably ridged vertically, 
so as to interlock and also so as to prevent their lateral displace- 
ment. 
In some of the specimens of Agelacrinus the lateral cover- 
ing plates differ considerably in size and form, some of them being 
longer and narrower and approaching the palisade-like effect of 
Streptaster. In these cases the conspicuous elongation is in all 
cases along the inner or concave curvature of the distal parts of 
the rays. This projects the median line of the ambulacral ray 
farther toward the periphery of the theca, and perhaps enlarges 
the effective feeding area of the animal. 
In Streptaster, the palisade-like elongation of the covering 
plates receives its greatest expression. Here (plate I, Fig. 7B, 
plate IV, Fig. 3), only two series of covering plates are in evidence, 
one on each side of the median line of the ray. Here, also the tips 
of the plates interlock by a prolongation of the proximal side of 
the covering plate into an angulation which fits into a recession 
located more distally, on the opposing plate. This interlocking 
may, at first glance, be obscured by the truncated appearance of 
the tip of the plates, but is shown by all well preserved specimens. 
If any median or intercalated covering plates were present, the 
latter must have been very small, and at least have not been 
identified so far. For the present, Streptaster is regarded as 
related more closely to Agelacrinus pileus than to any other known 
Ordovician species, but it is sufficiently distinguished by the 
presence of a mosaic of small polygonal plates, quite irregularly 
arranged, in the interambulacral areas. 
In Thresherodiscus ramosa the median or intercalated cover- 
