66 The Ohio Journal of Science [Vol. XIX, No. 1, 



shape is preserved in most of the fossils, it must have been 

 capable of some, if not great, vertical extension. The over- 

 lapping of so many rows of heavy plates (in A. cincinnatiensis 

 and Streptaster at least) for their full vertical dimension would 

 give a lateral support to the disc out of all proportion to the rest 

 of the skeleton. May it not be possible that in the living 

 cystid these plates did not overlap vertically to so great an 

 extent as in the fossil so that the individual could be extended 

 upward for a greater distance from the point of support. This 

 would give more visceral space. 



In a fragment of A. cincinnatiensis Plate II, Fig. 2, there are 

 eleven plates side by side in this inner portion of the peripheral 

 ring. Seven of these show at the surface and the other four are 

 covered. The height of these plates measuring up from the 

 substratum and beginning on the side toward the disc is as 

 follows: 1, 1.5, 2, 1, 1.5, 1.5, 1.25, 1, .75, 1, 1, in mm. Then 

 follow three rows of plates, the marginal zone, decreasing to 

 about .5 mm. in height. Adding the measurements of the 

 lower plates there is a total height of 12.5 mm. If these plates 

 were in a membrane the contraction of circular muscles might 

 have projected the animal out to the height of 12.5 mm. 

 (disregarding the over-lap). This section is shown drawn 

 with camera lucida on Plate VII, Fig. 46. 



The presence of a heavy muscular wall would explain the 

 perfect condition of the peripheral ring as well as if it (the 

 peripheral ring) were assumed to be rigid, for the plates, if 

 imbedded in a muscular wall, would be held in a definite position 

 with reference to each other for some time after the death of the 

 animal. Often the positions of the plates in the ring indicates 

 that they have been permitted to drop gradually downward. 

 The bottoms of the plates are found wedged together, while 

 the tops flare outward and inward. This wedging of the plates 

 of the inner part of the peripheral ring shows even better in 

 specimens of the genus Streptaster, and Streptaster may have 

 been more extensible than Agelacrinites. 



Most of these rim plates had processes on their aboral 

 surfaces, the larger plates as many as three processes and the 

 smaller marginal plates one. These processes are shown in 

 photographs 21, 22, 23 oi A. austini var lawshe. Not all the 

 plates are sufficiently well preserved to show such processes, 

 but whenever conditions are favorable the processes are found. 



