122 NEW YORK STATE MUSEUM 



following floor plates, these differences can hardly be attributed to 

 either age or difference in aspect. U . p u 1 c h e 1 1 a seems to be 

 the more primitive form and one in which the peristomial floor 

 plates could move slightly inward and function as jaws. In 

 U . medusa the ring seems to be too solid to allow such move- 

 ment and the projecting ends of these jaws were therefore partially 

 lost. 



In plate lO, radii A to D inclusive, portions of the oral surface of 

 these plates which are still buried in sediment suggest podial open- 

 ings. With the permission of former Director R. W. Brock,, 

 interradius (d) was gradually cut down and a series of stereograms 

 made of it during the process. Our plate represents the last of 

 these. This cutting involved two peristomial floor plates. The 

 floor plate of arm D has lost the distal excavation, seen in other 

 plates but still shows a portion of the proximal excavation. In 

 arm E the further cutting down of the plate next interradius (d) 

 has demonstrated the fact that these pits were not podial openings. 

 The proximal pit seems to have served as a muscle pit for an 

 adductor which assisted in drawing inward the first or interradial 

 pair of cover plates. The latter are seen to have formed a very 

 effective outer jaw (note particularly interradius a) for retention, 

 crushing, or thrusting captured specimens into the oral cavity. This 

 specimen shows an oral armature of both types, floor plate and 

 cover plate. The latter is of course the more primitive form. 



Turning now to plate 2, figure i, it will be noted first, that the 

 evidence against podial openings is conclusive. Regarding the 

 projection of medial toothlike processes, the upper left corner of 

 this plate seems to shoAv a projection 1.21 times the outer face 

 of the plate but the arm is here viewed more from the side and 

 plate I, figure i, shows some displacement of this pair of floor 

 plates. 



The proximal faces of a pair of peristomial floor plates are 

 shown in plate 2, figure 2. The outer edges of these faces measure 

 about .6 of their long or lateral diameters, while their inner faces, 

 due to thickening of the oral surface, measure .8 of this diameter. 

 The oral surface of each plate appears to have a shallow radially 

 placed channel. In plate i, figure i, the left channel of arm C 

 contains an apparently pear-shaped ossicle which, as it lies between 

 a pair of spines and the plate and seems to fit the excavation, may 

 belong to the specimen. The exposed end of this ossicle shows 

 longitudinal striae (or cleavage planes) and a suggestion of a 

 central, axial pore. It presents an appearance somewhat like that 



