BRACK lOPO DA. 4UI 
Oct Ills tiiceriaiiii.l 
partially occupied by a deltidium. Surface with thirty to thirty -six radiating, equal, 
but sometimes unequal, simple, subangular costae, with a linear elevation occupying 
the depressions, all crossed by exceedingly delicate concentric lines of growth. 
Ventral valve strongly convex, subangular, with the greatest elevation on the 
umbo. Cardinal area very wide, more or less convex and elevated. Delthyrium in 
the apical third occupied by a flat, concave or convex deltidium, which extends as 
well defined linear ridges along the entire length of the walls of the pedicle opening. 
Hinge teeth strong, supported by well developed, excavated dental plates, which 
join the outer elevated margin of the rounded muscular area. Diductor muscles 
occupy the greater portion of the muscular area, the anterior margin, of which is 
slightly thickened and unites with the two large, diverging vascular trunks. The 
adductor scars are two slender depressions situated between the diductors and sepa- 
rated by a central linear elevation, the whole being drawn out anteriorly into a 
very narrow and short septum. Genital markings close to each side of the muscular 
area. Exterior to these are numerous markings of the vascular system. Peripheral 
margins of valves marked by radiating, short and strongly elevated costae, each with 
a central furrow. 
Dorsal valve nearly flat, slightly elevated at the beak; from this the point of 
greatest elevation, the surface slopes gradually into the broad, scarcely perceptible, 
rarely well defined, median sinus. Cardinal area wide, flat, divided by a triangular 
delthyrium, as broad as long, and more or less covered by a convex chilidiura, the 
anterior margin of which is concave. Deltidial cavity occupied by a thin, much 
elevated, smooth cardinal process. Crural plates broad, strongly projecting inter- 
iorly, their bases converging and joining a low, broadly rounded, median septum, 
which becomes obsolete at about the mid-length of the valve. On each side of this 
septum are two pairs of adductor scars, the posterior pair well defined. Vascular 
sinuses numerous, occupying the entire postero-lateral surface of the interior. 
Orthis costalis Hall,* of the upper beds of the Chazy group of New York, is 
closely allied to 0. tricenaria. Remains of the former, however, are always more or 
less exfoliated, and this condition makes it difficult to point out satisfactory differ- 
ences between the two. Specimens figured by Mr. Walcott, and identified as 0. 
tricenaria, occur in the upper beds of the Pogonip group (probably equivalent to the 
Chazy group of New York and Canada), but are smaller than this species usually is. 
Such specimens, however, are also found near the top of the Trenton limestone at 
Minneapolis, Minn. After a careful examination of these specimens, the writers are 
unable to point out characters by which they can be separated from 0. tricenaria^ 
This variety was doubtfully identified by Dr. White {o^. cit.,) as 0. plicateUa Hall; 
but his illustrations show it to be the species cited above. 
*Pal. Ohio, vol. ii, p. 78. 
