420 
Aug. F. Foerste 
wider, and there was no indication of lateral grooves. The basal 
extensions of several of the lateral covering plates, however, were 
in evidence, and apparently extended beyond the lateral margins 
of the floor plates. However, since all the floor plates are badly 
weathered, further evidence is needed on this point. While the 
basal extensions of the lateral covering plates undoubtedly pass 
beneath the margins of the adjacent interambulacral plates, it is 
very likely that these extensions are enclosed, from beneath, by 
the floor plates, the total width of which probably was greater 
than suggested by the specimen described by Miller and Faber. 
20. Floor Plates of Streptaster 
In a specimen of Streptaster, found in the upper part of the White- 
water member of the Richmond, about three miles west of Dayton, 
and regarded as a normal specimen of Streptaster septembrachiatus , 
Aliller and Dyer, the most striking feature is the extreme narrow- 
ness and considerable height of the ambulacral rays, and the tall, 
narrow lateral covering plates (plate I, Fig. 7B; also plate IV, 
Fig. 2) looking like an uninterrupted series of more or less vertical 
palisades. The enclosed ambulacral cavity is high, but evidently 
very narrowx The upper ends of the lateral covering plates are 
])lunt, as seen from above. The bases of the covering plates on 
opposite sides of the rays practically must be in contact with each 
other. This is seen readily on viewing the rays from beneath. 
All the rays are contrasolar. The lateral covering plates rest 
upon the floor plates — two lateral covering plates on each floor 
plate, one covering plate on each side of the floor plate. The 
widest floor plate is barely 1 mm. in width, and this leaves very 
little space for the ambulacral groove. All five of the ambula- 
cral rays are exposed on the lower surface of the oral face of the 
theca, and, from this point of view, in all five rays, the floor of the 
ray consists of a single longitudinal row of floor plates (plate I, 
Figs. 7A, B), thinning and overlapping in a distal direction, 
forming angles of about 30 degrees with the former flat base of the 
theca. It is evident that if the floor plates could be seen from 
above, the proximal end of one would be found overlapping the 
distal end of the next, as in all other Agelacrinidae in which the 
floor plates are known. The upper, proximal side of the floor 
