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| 
1884.] 587 [Cope, 
F, Cummins in the Permian beds of Texas. Parts of the jaws with two 
of its teeth are preserved, The lower jaw is distinguished from that of 
the D. compressus by its small transverse as compared with its other di- 
ameters. The ramus is quite compressed, and is not thicker at the inferior 
edge than the superior, and is slightly concave on the inner side. Its ex- 
ternal face is nearly vertical. The angle is rounded forwards, and there 
is no angle behind the cotylus, which is raised above the superior line of 
the ramus. ‘The cotylus is rather large, and has a shallow anterior supe- 
rior, and a posterior subposterior facet. There is no indication of a coro- 
noid process. The inferior edge of the ramus is swollen on the outer 
side, below the anterior border of the condyle, so as to mark with the 
thickened posterior edge of the ramus a fossa in the position of the mas- 
seteric. 
The teeth are pecular in the form of the root (Figs. 8-9). This part has 
no anterior projection, and the posterior portion is a flat, thin-edged plate, 
wider than long. It carries a button, but no notch, There is a minute 
median denticle, The form of the root is thus very different from that of 
the tooth of the D,. compressus (figs. 5, 7). 
Measurements. M. 
Depth of ramus.at cotylus (vertical).............% Nie crs OR 
if My " 120 mm. anterior to cotylus. .048 
Transverse diameter at the same point.........s....... «009 
Long diameter (oblique) of cotylus............ Gerda an QOL 
g 1 J 
anteroposterlor........4'. O11 
LLANSV OIG. iieiile elon eels . 87 
{anteroposterior .0048 
( transverse..... .006 
Diameters of base of tooth | 
Diameters of crown of lateral denticle 
I call this species Didymodus platypternus. Should the name Didymodus 
be found hereafter to apply to species of Pleuracanthus, the latter generic 
name must be used for this species. 
Tit. Hisrorican. 
Tn 1837 Prof. Agassiz (Poiss. foss., iii, 66), described a spine which 
he believed to have belonged to a fish like the sting-rays, as Plewracanthus 
levissimus. The only example'was obtained from the Dudley Coal field. 
In 1845 Prof. Agassiz (Poiss. foss., iii, 204), made known certain 
teeth, which he referred to sharks of the family of Hybodonts. ‘Two spe- 
cies were distinguished, D. gibbossus and D. minutus. Both were obtained 
from the English Coal measures. 
In 1848 Prof. Beyrich (Berichte vernandl. k. Preuss. Akad. wiss., 
1848), proposed the generic name Xenacanthus for a German Carbonifer- 
ous form, referred to Orthacanthus by Goldfuss (1847), but which ap- 
proached nearer to Pleuracanthus. 
In 1849 Dr, Jordan (Jahrbuch fiir Min, u. Geol., p. 843), described, 
under the name Z?iodus sessilis, a form subsequently ascertained to be 
identical with the Xenacanthus. 
