65 



upper side, a secondary radial, which bears an ainbulacral open- 

 ing, and if this were all, the ray would bear two arms, and the 

 specimen would not belong to a twenty- armed species. But the 

 second plate, in the series of three, is axillary and if the ray were 

 straightened it would be the third primary radial, and the ray 

 would have four arms. This may be seen on the left of figure 25 

 and on the right of figure 24, as the two figures may be put to- 

 gether, with a slight overlapping. However, when you look at 

 figure 24 you can see, on the right, the series of three radial 

 plates, apparently occupying an interradial position, and a radial 

 series running off to the right that looks as if it might be the 

 support of the two ambulacral openings that we have just given 

 to the ray opposite the azygous area. This leaves in doubt the 

 question whether the ray opposite the azygous area, in the nor- 

 mal condition, had two ambulacral openings or four. If it had 

 four the species possessed, probably, twenty arms. Looking at 

 figure 24, you will see three ambulacra! openings above an inter- 

 radial area. If the ray were straightened and the two ambulacral 

 openings, on the right, given to the ray opposite the azygous area, 

 the ray would take these three openings above the interradial area, 

 and the ray would then have five arms. Probably this is where 

 the specimen is dwarfed and the two openings, on the left, should 

 be crowded over to the next ray, and the single opening should 

 be represented by two openings, and then there would be twenty 

 arms, in the species. If, however, this belonged to B. consan- 

 guineus, then the species was as variable in form, as we have 

 found B. variabilis and B. cognatus to be. As a matter of course, 

 where ambulacral plates are over interambulacral areas, the am- 

 bulacral canals curved so as to unite the circulation through the 

 regular ambulacral channels. From the appearances, we are in- 

 clined to believe that this specimen belongs to a twenty armed 

 species, and certainly, in a normal condition, possessed as many 

 as nineteen arms. The injury or abnormal development did not, 

 as we think, increase the number of ambulacral openings to the 

 vault, the tendency, if in any direction, would be to decrease them. 



Found in the Burlington Group, at Burlington, Iowa, and now 

 in the collection of S. A. Miller. 



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