Fauna of the Morrow Group 
101 
Horizon and locality. Hale formation: East Mountain, Fay- 
etteville, Arkansas (Stations 136 and 137). Brentwood lime- 
stone: near Fayetteville (Stations 134, 135, 140, 147, 152, and - 
153), and Brentwood (Station 145), Arkansas. Morrow for- 
mation: near Ft. Gibson, Oklahoma (Station 301). 
Pentremites rusticus Hambach 
Plate III, figures 3-6 a. 
1903. Pentremites rusticus. Hambach, Trans. Acad. Sci. St. Louis, vol. 
13, p. 54, text fig. 15. 
Chester limestone: Washington County, Arkansas. 
Description. Body sub-cylindrical, diameter and height sub- 
equal, diameter near upper extremity nearly as great as that 
near base, the greatest diameter occurring near the mid-height 
of the body; ambulacral plates nearly as long as the height of 
the body, the base flattened or moderately concave, the basal 
plates small; length of ambulacral plates 3% oi* 4 times their 
width, ambulacral pores about 17 in 5 mm. ; deltoid plates com- 
paratively small, their width about two-thirds their length in 
adults ; inter-ambulacral spaces flat or moderately concave, 
strongly elevated above the ambulacrals, the elevation being at 
a maximum at the apex of the deltoids and decreasing toward 
the base of the body; the margins of the plates bordering the 
ambulacral notches abruptly and strongly thickened, surface of 
plates finely striated parallel to their margins. 
Dimensions of Three Individuals. 
Height 
Maximum diameter 
Length of ambulacral plates 
Maximum width of ambulacral plates 
Length of deltoid plates 
Maximum width of deltoid plates 
Length of basal plates, from center of basal 
attachment scar 
Average number of side plates in 5 mm 
.17.9 
mm., 
17.3 
mm.. 
9.9 
mm. 
.18.0 
mm.. 
17.0 
mm.. 
9.5 
mm. 
.17.7 
mm., 
16.3 
mm.. 
9.0 
mm. 
. 4.7 
mm.. 
4.8 
mm.. 
3.5 
mm. 
CO 
mm.. 
4.7 
mm.. 
2.7 
mm. 
. 3.5 
mm.. 
3.0 
mm., 
1.0 
mm. 
. 4.5 
mm.. 
4.4 
mm.. 
1.7 
mm. 
17 
17 
17 
Remarks. In comparison with P. angustus, this species is 
characterized by its barrel-shaped, rather than conical, body, 
the strong elevation of the interambulacral plates, the compara- 
tively small deltoids and broad ambulacrals, and the equality 
between height and diameter. The two species, although both 
