344 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
Orthambonites on the other hand always has both valves convex although the 
pedicle valve is usually the deeper of the two. The palintrope of Orthambonites 
is generally short and strongly curved, often so strongly curved as to simulate 
the orthocline position. 
The later members of Hesperorthis are generally provided with a pseudo- 
deltidium. Usually this structure in the Ordovician members is located at the 
apex and covers but a small portion of the delthyrium. This pseudodeltidium 
is usually flat in profile and depressed slightly below the margins of the del- 
thyrium. In the earlier members of the genus the pseudodeltidium is lacking or 
has not been observed. In these shells identification with Hesperorthis is based 
on other internal details. 
Inside the pedicle valve of Hesperorthis the teeth are always small with small 
oblique fossettes ; the dental plates are short and of the receding type. The muscle 
area is usually cordate in outline, and the vascula media are like those of Ortham- 
bonites and Orthis. 
Inside the brachial valve the notothyrial platform is always well developed 
and thickened. The brachiophores are elongate and usually triangular in sec- 
tion and in this respect differ from Orthis and Orthambonites. The cardinal 
process is usually thickened at its base and often bears a narrow longitudinal 
furrow on the exposed face of the shaft. 
Hesperorthis, but without a pseudodeltidium, appears first in the middle por- 
tion of the Marmor, becomes abundant (with pseudodeltidium) in the late Mid- 
dle Ordovician, reaches its climax in the Richmond with a new gigantic species, 
and then declines into the Middle Silurian. 
HESPERORTHIS ANTELOPENSIS Cooper, new species 
Plate 269, C, figures 11-17 
Shell of about average size for the genus, subpentagonal in outline ; length and 
width about equal; side nearly straight, hinge width and midwidth nearly equal ; 
anterior margin narrowly rounded; anterior commissure faintly sulcate; costae 
rounded, numbering 32, and with 1 or 2 fine radial lines occupying the inter- 
spaces. 
Pedicle valve moderately convex in lateral profile, with the maximum con- 
vexity just anterior to the beak; anterior profile strongly domed, the median 
region narrowly convex and forming an indistinct fold; lateral slopes steep ; ears 
small, approximately a right angle. Interarea long and with a long, narrow del- 
thyrium. Pseudodeltidium small. Beak small and incurved. 
Brachial valve gently concave in lateral profile, somewhat more strongly con- 
cave in anterior profile; greatest concavity near the middle and extended an- 
teriorly to the anterior margin as a poorly defined sulcus. Flanks bounding sul- 
cus flat ; posterolateral extremities gently deflected toward the pedicle valve. 
Measurements in mm.—Holotype, length 16.3, length of brachial valve 13.0, 
midwidth 16.3, hinge width 16.6, thickness 8.3, height 8.7. 
Type.—Holotype: 124232. 
