PLATE ARRANGEMENT OF RHOECHINUS GRACILIS. 203 



column too far to the left when reversed for comparison with the outside 

 The seventh column of area /near the dorsal border is discontinuous in 

 one row where two plates, H H., of columns 5 and 8 come in contact. The 

 plates of column 7 are modified into pentagons of alternately opposite 

 position above and below the two heptagons. No other case of a break 

 just like this has been seen (see page 194). 



At the dorsal border the outlines of 5 genital and 2 ocular plates are 

 more or less visible. The genitals are large and similar to the genitals of 

 Palseechinus, as figured by Bailey (5). Pores exist in these plates of Rhoe- 

 chinus gracilis, but they are too irregular and poorly preserved to have 

 value attached to their number. The oculars are not well differentiated. 

 They apparently reach to the periproct. The plates of the interambu- 

 lacra in the dorsal portion assume a more or less rhombic form, as de- 

 scribed in Melonites (plate 3, figure 13), and Oligoporus, so that here again 

 this is the character of newly added plates. 



The outlines of plates in the cast are represented by comparatively 

 thick ridges, corresponding to the spaces between the plates. The ridges 

 are thicker than the spaces between the plates could have been, and this 

 is ascribed to enlargement from semiferruginous accumulation, just as in 

 Oligoporus danse (plate 6, figure 33) a similar enlargement is caused by 

 silicification. Each of the interambulacral plates has a number of papil- 

 lose projections, evidently corresponding to the position of spine bosses, 

 as shown in interambulacrum J. As the specimen is a cast of the out- 

 side, spine bosses should be depressions, not elevations. The reason of 

 the elevation of these areas is not understood. The spine tubercles of 

 Rhoechinus, as other members of the Melonitidse, are imperforate, not 

 perforated, as in Oidaris, etcetera. These elevations therefore cannot be 

 considered as enlarged casts of the pores of tubercles, as has been sug- 

 gested. The width of jthe ambulacra at the widest part is 5 millimeters, 

 that of the interambulacra at the same horizon is 14 millimeters. 



We have, then, in Rhoechinus the same method of arrangement of plates 

 and introduction of columns as has been previously described in Melo- 

 nites and Oligoporus. 



NO TES ON RHOECHIN US B URLINO TONENSIS. / 



Only two species of Rhoechinus have been described from American 

 formations — one, R. gracilis, which has just been considered, and another, 

 Rhoechinus hurlingtonensis, (Meek and Worthen) (30). Both species were 

 described under the genus Palseechinus. This latter species is from the 

 Burlington limestone, Burlington, Iowa. The species was described by 

 the authors from a specimen which showed the median portion of 3 inter- 

 ambulacral and 2 ambulacral areas. It was incomplete dorsally and ven- 



XXVI— Bull. Geol. Soc. Am., Vol. 7^ 1895. 



