BRAoHIOPODA. 781 
jugum is neither divided nor pectinated, while the spiral ribbon 
bears short spinules on the edge and face of the lamelle fronting 
the sides of the shell. 
(Type, Actinoconchus planosulcatus, Phillips (sp.). Carbon- 
iferous.) 
Subgenus Seminula, McCoy. 1844. 
(Plate 35, figs. 11-19.) 
Shells transverse, often elongate or ficiform; valves biconvex, 
the pedicle-valve with a median sinus over the pallial region, 
and the brachial valve with a corresponding ridge; both sinus 
and fold may be divided by a sharp median sulcus extending 
from the umbones to the margins. There is frequently 
evidence of asingle obscure lateral fold on each side of both 
valves. The umbo of the pedicle-valve is incurved and the del- 
tidial area is usually concealed ; the foramen, however, is exposed 
SS 
Figs. 313, 314.—The jugum of Seminula subtilita, Hall. 
as a circular or ovate aperture which encroaches on the substance 
of the valve. In the pedicle-valve the diductor muscular impres- 
sions are very faintly defined ; the adductor and pedicle impres- 
sions are as in the typical forms of Arnyris. In the brachial valve 
the hinge-plate is highly developed, its upper face being subquad- 
rate in outline, concave on the surface, the concavity deepening 
toward the visceral foramen which lies just beneath the beak ; 
not infrequently the foramen is closed by secretions of testaceous 
matter. The posterior flanges of the plate pass beyond the hinge- 
line and into the umbonal cavity of the opposite valve. The 
anterior face of the plate is erect and the anterior edge somewhat 
33 
