412 Aug. F. Foerste 
15. Trimerous Origin of Ambulacral System 
The ambulacral system of the Agelacrinidae evidently is of 
trimerous origin, although a pseudo-pentamerism has been super- 
induced upon the same. This pseudo-pentamerism is indicated 
by the fact that in all Ordovician species referred to Agelacrinus 
or Lepidodiscus , rays 1 and 2 (left posterior and left, respectively) 
and also rays 4 and 5 (right and right posterior) separate from each 
other at a greater distance from the center than ray 3 (anterior 
ray). A far more striking illustration of pseudo-pentamerism 
among the Agelacrinidae, however, is given by the Thresherodiscus 
ramosa (plate III, Fig. 3, and Fig. 1 on page 434). Here the 
trimerous origin of the rays is so evident that it is necessary to 
number the rays as in the pseudo-pentamerous species of Agela- 
crinus merely to admit of ready comparison with the latter. In 
Thresherodiscus , the ambulacral system originates at the center in 
three arms, left, anterior, and right, each of which branches dicho- 
tomously at a distance of about 2 mm. from the center of the peri- 
stomial area, further branching taking place, also in a dichotomous 
manner, distally. While it was scarcely necessary to discover this 
genus in order to demonstrate the trimerous origin of the ambu- 
lacral system of the Agelacrinidae, it must be admitted that 
Thresherodiscus offers a striking confirmation of this origin. 
16. Cover Plates of Ambulacral Rays 
The cover plates of the Agelacrinidae usually are well preserved. 
In Agelacrinus pileus (plate II, Figs. 1, 2), there are two series of 
cover plates, one on each side of the median line of the ray. In by 
far the greater number of specimens the proximal side of each 
cover plate is elongated into an acute spinous tooth which projects 
across the median line of the ray and interlocks with the opposing 
cover plate (plate I, Fig. 5A, B, C, also plate II, Fig.l). The 
latter also has a spinous prolongation on the proximal side, and 
both cover plates are curved concavely on the distal side of this 
spine, along the median line of the ray, so as to admit the inter- 
locking. In a few specimens, apparently of the same species, this 
spinous prolongation is absent or inconspicuous, and the cover 
plates on opposite sides of the median line of the ray, oppose each 
other like two series of V’s, with their apices alternating (plate II, 
Fig. 2). 
