PALAEONTOLOGY OF IOWA. 691 



pieces, which, in well-preserved specimens, are distinctly 

 striated in the longitudinal direction of the pores. 



Mouth central, pentagonal : ovarian apertures five, ovate, 

 somewhat closely arranged. 



SURFACE finely striate in a direction parallel to the margins 

 of the plates. 



This species resembles the P. sulcatus of RCEMER, but has the base more pro- 

 minent, and is more rapidly contracted above the widest part of the body. The 

 pseudambulacral areas are not so sharply and prominently margined by the edges 

 of the adjacent plates, the interradial plates do not extend as high, and the ovarian 

 apertures are comparatively smaller and closer together, and are oval or ovate, while 

 those of P. sulcatus are somewhat quadrangular. There are also other differences of 

 a less prominent character. 



Fig. 11 a. Lateral view of a specimen. 



Fig. 11 b. View of the summit of the same, showing the ovarian apertures, etc. 



Geological formation and localities. In the Kaskaskia limestone : Chester, 

 Illinois ; and near Huntsville, Alabama. 



PLATE XXV. FIG. 12 a, b. 



Pentremites cherokeeus : TROOST, MS. of Monograph; Catalogue, 1849, Proc. Am. Assoc. 



for the Advancement of Science, p. 60. 



Pentremites sulcatus : RCEMER, Mon. of Blastoideae, 1852, pa. 354, tab. vi, f. 10 a, b, c. 



BODY large, subglobose, subtruncate below; base slightly 

 projecting. Basal plates together forming a pentagonal mo- 

 derately convex disc. Radial plates broad, projecting some- 

 times almost rectangularly to the base of the pseudambula- 

 cral areas, deeply forked for the reception of the poral plates, 

 and rising abruptly in a thin sharp margin above and be- 

 coming more prominent at the apex. Interradial plates small, 

 lanceolate, usually as high as the truncate summit, and often 

 extending above it in acute points. Pseudambulacral areas 

 elongate, gradually widening above, concave, the two sides 

 flattened and regularly sloping to the median line, somewhat 

 deeply inserted in the forks of the radial plate : from forty 

 to fifty poral pieces on each side, marked down the centre of 

 each range by a suture-line and minute pores. Mouth central, 

 pentagonal : ovarian apertures somewhat quadrangular. 



