47 
Umbonal region of moderate prominence. Internally the prominent, 
triangular muscle area is bounded posteriorly by diverging ridges and 
divided medially by a short, broad septum. Teeth of moderate size, 
supported in youth by short dental lamellae, which later disappear. Del- 
thyrium apparently open. 
Brachial valve arched from beak to front, rather strongly in umbonal 
region, much less from the middle of the valve forward; transversely it is 
rather uniformly convex. Greatest depth of valve about the same as that 
of the pedicle, but anterior to the middle instead of posterior. Internally, 
the hinge-plate lodges medially a small cavity, the spondylium, and laterally 
the dental sockets; this plate is supported by the median septum which 
extends almost half the length of the valve, becoming narrower and lower as 
it advances. The outer edges of the hinge-plate are apparently crenulated. 
The entire surface of both valves is marked by fine and a few coarser 
concentric growth lines and by very fine radial striae; about five of the 
latter occupy the space of 1 mm. There are also from three to five low, 
broadly rounded, radiating plications in the median sinus and upon the fold, 
but none upon the lateral slopes of the valves. Though the typical form has 
inconspicuous, obscure median plications, other specimens with all the other 
characters of this, both external and internal, have no plications; we are, 
accordingly, inclined to include these latter specimens within the species. 
Remarks . The radial strise place this Liorhynchus shell within Weller's 
genus Paraphorhynchus. P. transversum Weller from the Kinderhook of 
Iowa and Illinois, though similar internally, except for the presence of the 
long, supporting dental lamellae, is wider than long, with plications upon 
the lateral slopes. P. elongatum Weller from the Kinderhook of Missouri 
differs internally and has, moreover, plications covering the entire shell. 
The absence of all plications upon the lateral slopes and their faint develop- 
ment or at times their entire absence upon the median part of the shell, 
together with the presence of dental lamellae in the young shell only, 
argue a much later period of development and hence, very probably, 
a later period of earth history for their existence. Some individuals retain 
the dental lamellae longer than others. 
Locality and Horizon. In the Minnewanka region in the Permian 
of section 1-14 (C); in the Pennsylvanian of section 2-12 (R). 
Genus, Dielasma King 
Dielasma chouteauensis Weller 
1914. Dielasma chouteauensis Weller, Geol. Surv., 111., Mon. 1, p. 257, 
PI. 32, figs. 1-17. 
Remarks. Our form is apparently similar to those described by Weller 
from Missouri. The faint radiating costae which are doubtfully said to be 
present were not noticed upon any of ours. This species may be recog- 
nized by its depressed convex shell; the greatest thickness is somewhat 
more than half its greatest width. In D. formosum the maximum width 
and thickness are almost equal. Internally the muscle-bearing platform 
bears a single cavity beneath it, whereas in D. formosum there are two, 
separated by a longitudinal median ridge. 
Locality and Horizon. Mississippian (Chouteau) of Missouri. In 
the Minnewanka region in the Lower Mississippian of section 2-22 (c), 
23 (C), 24 (r), 25 (c), 26 (r). 
10277-41 
