PLATE ARRANGEMENT IN OLIGOPORUS DANyE. 197 



are preserved. These spines (plate 6, figure 32) are long, cylindrical and 

 swollen at the base. On some, faint longitudinal striae were seen, which 

 are apparently original ornamentation. The spines are longer than in 

 Melonites multiporus (plate 2, figure 1), and do not show the bulging 

 which is characteristic of that species. Probably the spines originally 

 were longer than here figured, as they terminate distally in a blunt end 

 w^hich suggests a more elongate form when perfect. Another specimen 

 of Oligoporus danas, in Yale University Museum, shows an interesting con- 

 dition of fossilization of ambulacral plates. This specimen (plate 6, fig- 

 ure 33) is silicified ; but instead of the plates being silicified in parts of 

 the test the spaces between the plates and the pores have been the seat 

 of deposition of silica, as shown in plate 5. It is well known that 

 silicification enlarges parts by accretion, so here we get a silicious ridge 

 representing the space between plates which is much wider than the 

 space between the plates themselves could have been, and the ambulacral 

 pores which are represented by elevated plugs are larger than the pores 

 in normal plates of the species. This specimen is of interest as throwing 

 light on the structure of a specimen of Rhoechinus gracilis described in 

 a succeeding chapter. 



The ambulacrum of Oligoporus danse (plate 6, figure 31) was described 

 under the consideration of Oligoporus coreyi (page 190). 



Turning to the excellent figure of Oligoporus danae published b}^ Meek 

 and Worthen (30), the interambulacrum of which is reproduced in plate 

 6, figure 34, we find still further confirmation of the fact that the method 

 of plate arrangement in Oligoporus is like that oi Melonites. This figure 

 of Oligoporus, it should be stated, is the only one previously published of 

 any species in the family which at all adequately represents the method 

 of plate arrangement and the introduction of new columns. Rather curi- 

 ously the authors, while giving such an excellent figure, did not in their 

 text make any mention of this arrangement ; also, as published by them, 

 this figure was inverted, with what we now know to be the ventral border 

 uppermost. This is not at all strange, as the specimen did not show 

 genital or ocular plates, which would have indicated the correct orien- 

 tation, and deducting orientation from plate arrangement as here de- 

 scribed was unknown. The figure as here published is modified from 

 the original figure by omitting ambulacral plates, introducing letters^ 

 numbers and dotted lines to accentuate columns, and reversing the orien- 

 tation in accordance with the new view. 



Mr Jaggar, in his studies of the interambulacrum oi Melonites multiporus 

 (plate 3, figure 12) compared the plate arrangement in that specimen 

 with that of this figure of Oligoporus danse; therefore the comparison as 

 described should be credited to him. This figure at the ventral border 

 has a row representing 5 columns of plates, below which area it is not 



