REPORT OF THE DIRECTOR I9IS 121 



conception of the genus and also represents what the writer believes 

 to be a new species. 



Urasterella medusa, new species 



(Plates 1-6) 



Peristomial ring heavy. Axillary inframarginal prominent, its 

 long axis parallel with the ring. First floor plates (peristomial) 

 with radial diameters equal to the sum of those of the three follow- 

 ing floor plates. Arm marginals with long axes set parallel with 

 the ray. Supramarginals and axial columns consist of prominent 

 plates. All plates of the apical skeleton bear paxillae. Infra- 

 marginals and cover plates also bear paxillae but those of the latter 

 plates are small and delicate structures with short or modified 

 pedicels. 



In U. pulchella (Billings), to which this species is most 

 closely allied, the axillary inframarginals have their long axes 

 placed radially ; the first floor plates have radial diameters but twice 

 as great as those of the following floor plates ; and the infra- 

 marginals are placed with their long axes perpendicular to the ray. 

 Other differences will be noted in the more detailed description and 

 comparison which follow. 



Raymond's figure (loc. cit. ) and those here given are of the 

 holotype. The specimen was collected by Dr C. D. Walcott from 

 the Trenton limestone at Trenton Falls, N. Y., and is now in the 

 collection of the Museum of Comparative Zoolog}^ Harvard Uni- 

 versity. 



Description of the Preserved Structure of three American Fossil 

 Sea Stars Referred to the genus Urasterella, McCoy 



Floor Plates of the Peristomial Ring 



These plates are oppositely placed. In U . medusa (plate 2, 

 figure I, apical aspect) their average radial length is 1.2 of their 

 transverse diameter (measured along the edge of a distal face) 

 and the exposed edges of their inner (common) faces is 1.15 of 

 their outer. In U. pulchella (plate 10, oral aspect of 

 plesiotype) the average radial length of these plates is but .8 of 

 their transverse diameter while their inner edges are 1.5 the length 

 of the outer. As the width of the floor of the food groove is prac- 

 tically identical in the two specimens, and as the plates measured in 

 the plesiotype (those next interradius d) were cut down to about 

 half their thickness and allowance also made for overlap of the 



