Fauna of the Morrow Group 
185 
Spirifer opimus Hall 
Plate XII, figures 7-7c. 
1858. Spirifer opimus. Hall, Geol. Iowa, vol. 1, pt. 2, p. 711, pi. 28, figs. 
la-b. 
Coal Measures: Ohio; Maryland; Iowa; etc. 
1883. Spirifera opima. Hall, 2nd Rep. N. Y. State Geol., for 1882, pi. 
(31) 56, figs. 4-7. 
Coal Measures: Iowa; Brazil, South America. 
1894. Spirifer opimus. Hall and Clarke, Int. to Study of Brach., pt. 2, 
pi. 27, figs. 12-13. 
Coal Measures: Iowa. 
1895. Spirifer opimus. Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, pi. 31, 
figs. 4-7. 
Coal Measures: Iowa; Bomjardim, Brazil. 
Although this species has been many times included in the 
synonymy of *8. rockymontanus it is believed that the two forms 
are quite distinct and that Hall’s species is a valid one. In the 
material at hand it is represented by a number of specimens, 
some of which are nearly complete. The 'original description 
needs alteration in but one particular to fit these individuals, as 
obviously Hall intended his statement, “Surface marked by from 
eight to ten simple, abruptly elevated plications,” to mean that 
number on either side of the fold and sinus. The number of 
plications in the sinus and on the fold is not, however, limited 
to three in the material at hand but depends on the size of the 
individual and may be as great as six. 
S. opimus may be distinguished from S. rockymontanus by the 
normally larger and more regular lateral plications which are 
always simple in the former, as well as by the more transverse 
outline of the latter. In S. opimus the plications of fold and 
sinus are somewhat less strongly elevated than those on the 
lateral slopes of the valves while in Marcou’s species no differ- 
ence in strength of plications is discernible. 
The dimensions of the largest individual from the Morrow 
formation referred to this species are: length of brachial valv3 
from hinge-line to anterior margin, 15.0 mm. ; length of pedicle 
valve from umbonal region to anterior margin of mesial sinus, 
21.9 mm. ; length of hinge-line, 22.8 mm. ; greatest width of shell, 
24.5 mm. ; maximum thickness of shell, 15.3 mm. 
Horizon and locality. Brentwood limestone: near Fayette- 
ville, Arkansas (Stations 134, 150, and 152). ?Kessler lime- 
