Agelacrinidae and Lepadocystinae 401 
director of the Geological Survey of Canada, under whose auspices 
he was permitted to make the investigations which led to the 
discovery of the unique specimens here described under the new 
generic terms, Thresherodiscus, and Brockocystis. 
2. Character of Surface Used for Support by 
Ordovician Agelacrinidae 
The Ordovician species referred to Agelacrinus occur chiefly on 
Rafinesquina. Among 17 specimens of Agelacrinus cinncinati- 
ensis, 16 occurred on Rafinesquina^ and 1 appeared to have been 
unattached. Among those found on Rafinesquina, all were 
attached to convex surfaces; 13 on the exterior of the pedicel 
valves, and 3 on the interior surface of brachial valves. Among 
26 specimens of Agelacrinus pileus, 23 were found on the exterior 
of the pedicel valves of Rafinesquina, 1 appeared to have been 
unattached, and 2 were attached to bryozoans: one of these 
bryozoans was Homotrypa fiahellaris, and the other, Heterotrypa 
frondosa. The types of Agelacrinus holhrooki, Agelacrinus warren- 
ensis, and Agelacrinus vetustus occur on Rafinesquina. The type 
of Agelacrinus faheri (plate III, Fig. 4) occurs on the brachial 
valve of Hebertella alveata. The Ordovician Agelacrini probably 
settled on any convenient convex surface on the sea bottom, and 
the valves of Rafinesquina were preferred on account of their 
comparative smoothness. 
3. Orientation of Agelacrinus pileus on Sloping 
Supports 
In the case of the more convex species, such as Agelacrinus 
pileus, with one dextral and four sinistral rays, the orientation 
of the specimens appears to have been chiefly such as to place the 
anal interambulacral area on the right side of the sloping surface. 
For instance, among 24 specimens of Agelacrinus pileus, 14 had this 
anal interambulacral area on the right side of the specimen; in 4, 
this area was directed diagonally downward toward the right; in 
3, it was directly downward and toward the left; in 1, it was 
directed toward the left; and, in 2, it was directed diagonally 
upward and toward the left. The direction in which the surface, 
upon which each individual rested, sloped, was determined in 
each case by noticing in what direction the specimen of Agelacrinus 
