Fauna of the Morrow Group 
241 
Genus SPHAERODOMA Keyes 
Sphaerodoma sp. 
Plate XV, figure 26. 
This species, represented by two very imperfect specimens, 
is referred with considerable confidence to Sphaerodoma al- 
though its specific relations cannot be determined from the 
material at hand. In general, it resembles medialis (Meek 
and Worthen) but is only about half the size of the figured type 
of that species. From the description of one of the Manzano 
specimens described by Girty^® as aff. medialis it would 
appear that a close resemblance must exist between that form 
and the Morrow individuals. 
Horizon and locality, Brentwood limestone: near Fayette- 
ville, Arkansas (Station 135). Morrow formation: near Cho- 
teau, Oklahoma (Station 302). 
CEPHALOPODA 
ORTHOCERA TID^ 
Genus ORTHOCERAS Breynius 
Orthoceras sp. 
Plate XVI, figures 7, 8. 
Description. Shell slender, straight, without annulations or 
longitudinal ridges, circular in cross-section, tapering compara- 
tively rapidly, one specimen enlarging from a diameter of 2.3 
mm. to one of 4.4 mm. in the space of 15 mm. ; septa simple, 
concave, 4 or 4% occuring in the length of one diameter; 
siphuncle near, but apparently never at, the center, small, its 
diameter equal to about one-eighth that of the shell; surface 
devoid of ornamentation in the material at hand. 
Remarks. Fragments of several individuals of this species 
are in the Morrow collections and represent forms ranging in 
diameter from 1.7 mm. to 13 mm. In none is there any devia- 
tion from the circular cross-section nor any evidence of surface 
markings though the preservation is such in many cases as 
to retain what is believed to be the shell surface. 
This is probably the form which has been variously identified 
as O. okawerise or 0. rushense, but it is impossible from the 
published figures and descriptions to confirm the suspicion that 
