436 
Aug. F. Foerste 
eral edges of these lateral covering plates overlap each other, the 
distal side of one plate being covered by the proximal side of the 
next succeeding plate. The general form of the lateral cover- 
ing plates is elongate pentagonal, the tips alternating with the 
central covering plates. About six lateral covering plates occupy a 
length of 2 mm. This suggests the presence of at least two lateral 
covering plates along the side of each floor plate, probably decreas- 
ing to one on each floor plate as the smaller extremities of the 
last branches are reached. 
In one respect the structure of the rays of T hr eshero discus 
differs strongly from that of Agelacrinus cincinnatiensis, A. pileus, 
A . holbrooki, and probably also of other Ordovician species usually 
referred to Agelacrinus or Lepidodiscus. There is no known pro- 
longation of the lateral covering plates extending laterally beneath 
the adjacent interambulacral plates. 
The central covering plates of the ambulacral rays, or rather 
those parts of these plates which are visible, are very small and 
the definite determination of their arrangement is difficult. Along 
one part of one ray these central covering plates are arranged in 
two series, alternating both with each other and also with the tips 
of the lateral covering plates. However, there are other parts 
of the specimen in which the central plates appear to be more 
numerous than the lateral ones, and in which the arrangement, in 
consequence, appears less regular. 
The interambulacral plates differ conspicuously in size. Of 
these, the central plates (plate I, Fig. 8, ia) are much larger, dis- 
tinctly squamose, and imbricating, the distal end of one being 
overlapped by the proximal end of the next. Owing to the rather 
numerous branches of the rays, the primary interambulacral areas 
are so much divided and the individual parts so narrow that the 
large squamose plates along the center either form an approxi- 
mately straight row, or a more or less strongly zigzagging series, 
alternation being always conspicuous at the distal end of the 
series, where the large, central interambulacral plates merge into 
the numerous laterally elongated plates that belong to the inner 
band of the peripheral ring. The nearest approach to a single 
straight series of large plates is found in the interambulacral 
space starting at the point of divergence of the proximal parts of 
rays Nos. 1 and 2, and in that which starts in the angle between rays 
Nos. 3 and 4. The approximately straight part of these series is 
