426 
Aug, F, Foerste 
found among the Diploporita. Of course^ there are no diplopores 
among the Agelacrinidae, and there are no brachioles, so that the 
Agelacrinidae probably originated from an earlier stock, but one 
in which pseudo-pentagonal symmetry already was present. 
As regards the peristomial plates of the Agelacrinidae, those of 
Thresherodiscus are the most primitive. In those forms, however, 
in which the number of peristomial plates was small, as in Agela- 
crinus pileus (plate I, Fig. 5B), the primitive arrangement appears 
to have been a series of five plates of which the posterior was the 
largest. The next larger in size were the two plates on the anterior 
side of the peristomial slit — ^the plate between the anterior ray 
and the right ray, and the plate between the anterior and the left 
ray. The smallest plates of the peristomial series were between 
the bases of the left and left posterior rays, and between the right 
and right posterior rays. 
While the earlier Amphoridea may represent the most primitive 
types among the Cystidea, it is evident that pseudo-pentamerism 
had developed among the Cystidea long before the Agelacrinidae 
deviated from this stock. 
The Edrioasteridae may have had quite a different origin from 
the Agelacrinidae, since their floor plates are arranged in two series, 
one on each side of the ray, which alternate along the median line. 
26. The Ornamentation of the Surface of the Thecal 
Plates 
The surface of the thecal plates of Agelacrinus cincinnatiensis 
and of Agelacrinus holbrooki is essentially smooth. In some of the 
specimens of Agelacrinus pileus, however, the surface is covered 
by numerous closely arranged very minute pits, seen only under a 
lens. In specimen No, 13268-I-a, belonging to the American 
Museum of Natural History, these pits are shown on every exposed 
surface, including the peristomial plates, the lateral covering 
plates, the interambulacrals, the plates belonging to the anal 
pyramid, and the plates forming the inner band of the peripheral 
ring. Since the small marginal plates of the peripheral ring are 
not exposed, nothing can be definitely said regarding the latter, 
but probably the same minute pits are present at least on the 
larger ones of these marginal plates. 
