1912] The Ottawa Naturalist. 107 



Hudson interpreted the larger of these plates as an interradial 

 marginal from another specimen of this species, but the writer 

 is unable to accept this interpretation for two reasons. The 

 first is. that although the shape is much the same as that of one 

 of the interradial marginals, still there is an important difference. 

 This plate is pointed at the wider end, and evidently had a plate 

 resting against each of the two plane faces at that end. The 

 interradial marginals, on the other hand, are not pointed, but 

 have a plate resting directly against the end on a line with the 

 long axis. Secondly, the large plate has another fairly large 

 plate still in position, resting against one of the faces on the 

 larger end, and this plate is larger and of different shape from 

 any of the plates which abut against the inner faces of the inter- 

 radial marginals. Moreover, if these were plates foreign to this 

 specimen, they would not maintain their natural position in 

 relation to each other, but would be separated. It seems 

 probable that they are plates of the abactinal system of this 

 same specimen, and that they are not far from their original 

 position. A specimen of Palaaster matutina, Hall, in the Museum 

 of Comparative Zoology, shows interradial marginal plates of 

 this same form on the abactinal side. 



It thus seems probable, both from analogy with the specimen 

 of Urasterella pulchella here figured, and from the structure of 

 the specimen itself, that the "covering plates" of Protopalceaster 

 narrawayi, are really ambulacral ossicles exposed from the upper 

 side.. 



Explanation of Plate VI. 



1. Urasterella pulchella, (Billings). A specimen exposed 

 from the abactinal side, with nearly all the plates of the abactinal 

 skeleton weathered away, exposing the ambulacral plates. 

 Near the ends of two of the arms some of the ambulacrals are 

 lost , revealing the pits in the adambulacrals. On the arm running 

 to the left, a small patch of plates of the abactinal covering are 

 still to be seen in position, covering the ambulacrals. The 

 spinose marginals show but faintly in this photograph. The 

 figure is three times natural size, and the specimen, which 

 is from the upper part of the Trenton at Trenton Falls, New 

 York, is in the Museum of Comparative Zoology. 



2. Proto polo? aster narrawayi, Hudson. A photograph of 

 the holotype, showing the "covering plates," and the large 

 displaced plate which is now believed to be an interradial 

 marginal belonging to the abactinal size of this specimen. Note 

 the pointed inner end of this large plate and the smaller plate 

 still in position against one of its faces. This figure is 2.66 

 times natural size, and was made bv Professor G. H. Hudson. 



