124 NEW YORK STATE MUSEUM 



while the long axes of the plates measure but 1.2 mm. As the 

 cover plates, when in open position, rest partly on the outer beveled 

 faces of the floor plates (plate i, figure i, arm A) our measure of 

 the long axis of the latter in U . p u 1 c ,h e 1 1 a may be slightly 

 under the true length. If we increase this length, however, we 

 but increase the difference in form in the two species. We may 

 then safely say that the oral faces of these plates in U . p u 1 - 

 c h e 1 1 a are more slender than in U . medusa. We should 

 note also a change in form as we pass to more distal portions of 

 the arm. In plate 3, figure i, the sum of the radial diameters of 

 plates 2 to 8 inclusive is 3.4 mm, while plates 9 to 16 measure 4.1 

 mm. With this slight increase in radial diameters there goes a 

 decrease in transverse diameters and a marked thickening of the 

 floor apically. This thickening is already apparent in the third 

 floor plate of the left row in figure i. In both figures of our plate 

 3, this thickening is seen rapidly to bring about a marked convexity 

 of the apical surface of the arm floor. This thickening and the 

 concomitant convexity is still more manifest in the last floor plates 

 shown in plate i, figure 2. 



Turning now to the holotype of U. pulchella (plate 9, 

 figure 3) we find that at the sixteenth floor plate the medial thicken- 

 ing of the floor has given the inner faces of these plates a length 

 equal to that of their oral faces. These sections of arm A of 

 the holotype reveal the fact that at this distance from the mouth 

 the apical skeleton was in contact with and fitted the apical surface 

 of the arm floor. As evidence of this condition note, in the figure 

 referred to, the curve where the supramarginal at the left fits the 

 floor plate. In plate 5 the radials and supramarginals conform 

 closely to irregularities of the floor plates and pits left by the loss 

 of two plates of the radial series may be clearly seen. Both 

 figures of plate 3 show other decided imprints of apical plates 

 and in figure i, in the region of the fifth and sixth floor plates, a 

 single radial is still present in its pit and a similar pit lies over the 

 inner ends of the third and fourth floor plates. Plate i, figure i, 

 shows the solitary radial viewed somewhat from the side. From 

 the evidence given above we must conclude that the viscera did 

 not extend into the arms beyond the second floor pieces. 



The evidence given is also against the assumption that the genus 

 possessed internal ampullae and podial openings. The exposed 

 apical surfaces of the floor plates in our figures of U . medusa 

 give positive proof that such was not the case. U . medusa is, 

 however, not alone in yielding this evidence. In 191 5. plate II, a 



