184: R. T. JACKSON STUDIES OF PAL^ECHINOIDEA. 



several species (table facing page 242). One species, M. dispar, has but 

 6 columns ; another, M. giganteus, has 12 columns, and one, M. ethe- 

 ridgii, lias 12 or 14 columns of plates ; also, while Meloniies multijyorus has 

 10 columns of ambulacral plates as the feature of the species, it maybe 

 that some specimens may only show 8 columns throughout the area. In 

 this species, in one specimen figured (plate 5, figure 20) only 8 columns 

 of plates exist at the ambitus (as shown in the lower portion of the am- 

 bulacrum figured), whereas a little further dorsally (in the upper por- 

 tion of the ambulacrum figured) 10 columns may be counted. These 

 two last added columns in this specimen are added later than usual, and 

 it is conceivable that in some cases they might not be added at all. 



Consideration of Oligoporus and Comparisons of the same with 



Melonites. 



descriptiox of oligoporus missouriexsis, sp. nov, 



Plate 9, figures 50-52. 



A fine new S})ecies of Oligoporus has recentl}'' come to hand from the 

 Subcarboniferous of Webb City, Missouri. The exact horizon from which 

 this specimen came is somewhat uncertain, but Dr C. R. Keyes, chief 

 of the Missouri Geological Survey, kindly informs me that it is in all 

 probability from the Augusta limestone. To this species I would give 

 the name Oligoporus missouriensis, which is approi)riate in recognition of 

 the fact that this state has yielded such rich material for the elucidation 

 of the complex structure of Paleozoic Echini. The specimen is a silici- 

 fied cast free from matrix and is ver}^ slightly compressed. It is in the 

 collection of the Museum of Comparative Zoology, catalogue number 3078. 



The specimen measures 9 centimeters in height through the dorso- 

 ventral pole ; it is somewhat compressed, so that the dorso-ventral meas- 

 urement is slightly exaggerated ; greatest width through the ambitus in 

 the plane of compression, 9.6 centimeters ; width at right angles to plane 

 of compression, 7 centimeters. The outline of the test presents an even, 

 almost continuous, curved outline, the ambulacral and interaml)ulacral 

 areas presenting very little elevation beyond the outline of the whole. 



The ambulacra consist of 4 columns of low plates in each area ; width 

 of ambulacra at ambitus, from 1.4 to 1.6 centimeters, narrowing at the 

 dorsal area. Accessor}^ plates, as seen in Oligoporus danx (plate 6, figure 

 30), not present. Two pores are in each plate. The spinose projections 

 of the s[)ecimen which represent the ambulacral pores lie near the 

 middle of each half ambulacral area (plate 9, figures 50 and 51). This 

 is attributed to the f\ict that the specimen is an internal cast, and that 

 the pores pass toward the center of the half-areas in traversing the thick- 



