PLATE ARRANGEMENT IN OLIGOPORUS DANvE. 193 



with two columns on either side, as usual. This column originates in the 

 same row as it does in Melonites muUiporus (see discussion, page 162). The 

 interambulacral area is not preserved above this point. 



In a later period of growth, which represents as far as preserved the 

 interambulacrum of adult Oligoporus coreyi (plate 6, figure 28), there is at 

 the ventral border a pentagonal plate 4, representing the initial plate of 

 column 4. This column has at its origin two columns on the left and 

 one on the right (as seen by producing ventrally the plates which are 

 missing in column 1 at this point), which is the equivalent of two on the 

 right and one on the left as viewed from the outside. It is, therefore, a 

 left-handed column, as in the same column of area /of Melonites (plate 2, 

 figure 2). The fifth column is introduced by pentagon 5 in the third 

 row above 4, as in the other area just described. Pentagon 5 has a hep- 

 tagonal plate, H, on its right border (the left as viewed from without). 

 In the third row above 5 the sixth column is introduced by pentagon 6, 

 with a heptagonal plate on its right (left as viewed from without), the 

 normal position. At its origin column 6 has two columns on its left and 

 three on its right, the normal position, when the numbers are reversed for 

 comparison from without. Above pentagon 6, 6 rows of plates are intro- 

 duced without the appearance of a seventh column, and as the seventh 

 is introduced soon after the sixth in Oligoporus danse (plate 6, figure 34) 

 and all specimens of Melonites muUiporus and M. giganteus, there is in this 

 fact evidence for supposing that no more columns would originate, as 

 previously discussed. The inward curvature of the sides of the inter- 

 ambulacrum in its dorsal portion indicates that it is relatively near what 

 was the dorsal termination of the area, so that if any more columns were 

 present in the specimen when entire, they must have originated quite 

 close to the genital area, as seen in the ninth column of Rhoechinus 

 gracilis (area (7, plate 7, figure 36). 



DESCRIPTION OF PLATE ARRANGEMENT IN OLIGOPORUS DAN^. 



Oligoporas danse, Meek and Worthen, of the Keokuk group has been 

 described only by the authors (30) in their original publications on the 

 species. I am therefore able to add some new features to the present 

 knowledge of the species, as well as to make comparisons of its plate 

 arrangement with that of Melonites/^ 



A specimen of Oligoporus danse in the collections of the Museum of 

 Comparative Zoology (catalogue number 2997) shows nearly the whole 

 of an interambulacrum above the ventral portion, as well as the ambu- 



*The types of Oligoporus danos and O. nobilis are in the Worthen collection in the University of 

 Illinois, at Urbana, Illinois (see footnote, page 136). The types of Oligoporus bellulus, blairi and 

 sulcatus are in the private collection of Mr Wm. F. E. Gurley, of Springfield, Illinois, as I am in- 

 formed by that gentleman. 



