8 Field Museum of Natural History — Geology, Vol. IV. 



the plates appear as prominent nodes. These middle portions of the 

 plates are free from tubercles, but the parts of the plates covered by 

 the parallel lines and the plates of the petals are covered with minute 

 tubercles closely crowded together. The lines are much less prom- 

 inent on the plates of the ventral surface and the tubercles are larger, 

 with slightly sunken areoles and more evenly distributed over the 

 surface except on the longitudinal median band, which extends from 

 the peristome to the posterior margin. The peristome is situated at 

 the center of the ventral surface, surrounded by a floscelle, the bourre- 

 lets are prominent and the phyllodes are depressed and contracted 

 into narrow grooves, where they reach the peristome. The periproct 

 is supramarginal, but its form and exact position are unknown because 

 that portion of the posterior interambulacral area near the margin 

 is missing. The measurements of the type specimen, Cat. No. P 

 10347, are: Length 34.7 mm. width 26:7 mm., height 19.5 mm. 

 Allowing for the lateral crushing it is probable that the length and 

 width were originally about 32 mm. and 29 mm. respectively. 



It is to be regretted that other and more perfect specimens 

 of this species could not have been found and the writer has endeav- 

 ored to obtain such specimens by correspondence, but so far without 

 success. The specimen on which the description is based is crushed 

 laterally, and in three of the interambulacral areas and the region 

 about the peristome there are some plates missing. Three of the 

 ambulacral and two of the interambulacral areas are practically 

 complete. The entire petaloid portion of the dorsal surface is in good 

 condition, so that the only important character that is in doubt is the 

 exact form and position of the periproct. The five pairs of plates 

 above the margin in the posterior interambulacral area are missing 

 and the periproct was situated somewhere within this space. The 

 specific name is adopted on account of the hemispherical form of 

 the test, which distinguishes it to a marked degree from any other 

 species of the genus. 



Locality : The type specimen was collected by the writer in the 

 "white gullies" of the Ripley Group on the Patterson farm, about 

 three miles south of Pontotoc, Mississippi. 



