61 



uearly perpendicular. Each articulating facet forms more 

 than half a circle and has a diameter about one- half the di 

 ametcr of the plate and is slightly notched for the ambulacra! 

 canal. 



The angles for the reception of the regular interradials ai-e 

 .somewhat acute, and the interradials are longer than wide and 

 stand upright, so as to elevate the vault, and give to the 

 whole body a pentagonal, subglobose outline. The Hrst azy- 

 goiis ]ilate is not as long as the first regular interradials, but 

 a little wider, and bordered at the upper pai't of each side by 

 an interi'adial which gives to the azygous area greatei- width 

 than is found in the other areas, and it is surnu)unted with 

 two small plates that form part of the I'ini surroiuiding tiic 

 azygous opening. 



The vault is alu'uptly (>l('vated i)\'er the calyx so as to re 

 tain the pentagonal outline, with concave sides, and so as to 

 have a capacity greater than that of the calyx. It is convex 

 on top and covered with a few large, polygonal, subspinous 

 plates. The largest jilate is subcentral on the azygous side 

 and surrounded by nine plates. The azygous orifice is large 

 and situated between the ambulacral openings, though not ex- 

 tending quite so low. It is surrounded by eight plates. 



The calyx of this species or varietj', as the case, in fact, 

 may be, is not as concave below, as it is in P. formusiis, and 

 the calyx in that species is four times as wide as higli, while, 

 in this, it is only three times as wide as high. The sutures 

 are canaliculate in that species, and beveled in this. The angles 

 of the interradials ai-e more acute in this variety than in that, 

 and appearances indicate that the vault in P. formosas is lower 

 than it is in this. If, however, the vaults are alike, this is 

 mei-ely a varietal form of P. formosus, unless the difference 

 between the canaliculate and beveled sutures is of specific 

 importance: for the ditference in form may exist, in the same 

 species. If, again, this form sliall. in future, lie referred to 

 P. foj-iHositf, we are fully justified in publishing it as a varie 

 tal form, because we have a complete specimen differing in 

 some respects, from the type, so far as it was preserved. 



Found in the Burlington Group, at Sedalia, Missouri, and 

 now in the collection of F. A. Sampson. 



