184 
Kirtley F. Mather 
sinus, many of them simple but a few, more commonly those 
adjacent to the sinus, bifurcating. 
Brachial valve not so deep as pedicle, its beak small and more 
closely incurved, cardinal area inconspicuous ; mesial fold sharply 
defined in all stages of growth, originating as a single plication 
on the umbo which soon bifurcates and becomes more strongly 
elevated, other plications appearing on the slopes on either side 
of these two which ordinarily occupy the crest of the fold to 
the anterior margin, the number of plications on the fold there- 
fore depending upon the age of the shell, but in the majority 
of cases being about 8 ; lateral plications as on the pedicle valve. 
The minute surface markings are rarely retained but consist 
of fine radiating striae crossed by fine concentric, wavy, growth 
lines, the two sets of markings sub-equal in strength. 
Remarks. This shell, one of the most abundant in the Mor- 
row collections is characterized by the occasional bifurcations 
of the lateral plications in which it agrees with the larger speci- 
men figured by Marcou, the first of the two to be mentioned in 
his text as well as in the legend to the plate and therefore the 
type of the species. Spimfer opimus Hall is apparently a form 
ordinarily smaller than the adult members of this species and 
its lateral plications are invariably simple, as inferred from the 
figures and descriptions. It is not synonymous with S. rocky- 
montanus, S. boonensis, as described by Swallow and inter- 
preted by Girty^^, is also characterized by simple lateral plica- 
tions and although similar in outline to the forms with longer 
hinge-lines which are here referred to S. rockymontanus it is 
evidently distinct from them. 
Horizon and locality. Hale formation: East Mountain, Fay- 
etteville, Arkansas (Stations 136, 137, and 149). Brentwood 
limestone: vicinity of Fayetteville, Arkansas (Stations 134, 135, 
138, 140, 147, 150, and 154) ; Sawney Hollow, Oklahoma (Station 
210). Kessler limestone: near Brentwood (Station 144), and on 
East Mountain, Fayetteville (Station 209), Arkansas. Morrow 
formation: near Wagoner (Station 294), Choteau (Stations 295, 
297, 298, 302, 306, and 307), Hulbert (Station 299), Ft. Gibson 
(Station 303), and Gore (Stations 304 and 305), Oklahoma. 
