Silurian Fossils 
IS 
western form was termed Orthis flabellites in this list is unknown. 
In volume viii, New York Paleontology, on plate 84, Hall and 
Clarke figured Orthis flabellites-spania from the Niagara dolo- 
mites near Milwaukee, Wisconsin. 
Typical Orthis flabellites occurs also in the Osgood bed of Indi- 
ana, from which the following description is drawn up. It is 
characterized by the presence of 28 to 30 simple radiating plica- 
tions, separated by deep narrow grooves. Specimens with 25 to 
27 plications are not rare. The brachial valve is evenly convex, 
having a depth of 4 to 5 mm. in shells 21 mm. in length. The 
convexity of the pedicel valve depends on the height of the hinge 
area which varies from 3.5 to 6 mm., averaging at about 4 mm. 
From the beak of the pedicel valve the shell slopes with a very 
slight convexity toward the anterior and lateral edges of the shell, 
but owing to the height of the hinge area, this results in giving the 
valve a distinctly convex form, with the point of greatest elevation 
at or near the beak. The hinge area of the pedicel valve forms 
an angle of about 60° to 65° with the plane dividing the valves. 
The hinge area of the brachial valve forms an angle usually of 5° 
or 10°, rarely of 30°. The muscular scar of the pedicel valve is 
of an obovate form, the sides being distinctly outlined, and con- 
verging anteriorly; the anterior termination of the muscular scar, 
however, usually is indistinctly outlined. When distinctly out- 
lined anteriorly, the outline is seen to be reentrant at the anterior 
margins of the diductor scars, so that the anterior margin of the 
adductor scars lies at the rear of this angle. The adductor scars 
are linear in form, and about a millimeter in width, the entire 
muscular area having a width of 5 mm. 
Osgood bed: New Marion, Osgood, Big creek, Nebraska, in 
Indiana. 
Orthis flabellites-militaris, var. nov. 
The large form of Orthis flabellites found in the Clinton at the 
Soldiers’ Home, near Dayton, Ohio, and represented by figs. 12a 
and 12b on plate xiii, vol. i, of this Bulletin, differs chiefly in hav- 
ing only 20 to 24 plications, and in having a broad shallow median 
depression near the beak of the brachial valve, as well as may 
be determined from the specimens split out of the limestone. 
The pedicel valve is strongly convex, especially toward the beak 
which is distinctly incurved. 
Clinton bed : at Soldiers’ Home, near Dayton, Ohio. 
