ra! 
muscular fibres; the inner socket walls are considerably prolonged into the cavity of the shell, 
under the shape of projecting laminze, to the extremity of which free fleshy spiral arms may, 
perhaps, have been affixed. Under this shelly process a longitudinal ridge separates the 
quadruple impressions of the adductor, which on each side forms two deep oval depressions, 
placed obliquely one above the other, and separated by lateral ridges branching from the 
centre one.” (Davidson, Brit. Foss. Brachiopoda, Introduction, p. 102). 
The genus Orthis is very nearly related to Strophomena, from which the more typical 
forms may in general be easily separated by their usually greater convexity, and the rounder 
and shorter hinge-line. Two spevies of the genus, viz.: 0. Livia and 0. Vanuxemi have been 
recorded by Mr. Billings as occurring in the Devonian Rocks of Ontario, and both of these 
have come under my observation. 
84. Ortuis Livia (Billings). 
Orthis Livia (Billings), Canadian Journal, New Series, Vol. V. p. 267, Figs. 14-16. 
Orthis Livia (1) (Hall), Pal. New York, Vol. 1V., Figs. 4a—4e. 
“ Sub-orbicular or sub-quadrate ; length about eight-ninths of the width; greatest width, 
usually a little in front of the middle ; length of hinge line, one-half to two-thirds the width 
of the shell ; cardinal extremities rounded ; sides in most specimens somewhat straight, often 
sufficiently curved to give a circular aspect to the shell; front angles obtusely rounded ; front 
margin in general broadly convex, sometimes with a small space in the middle nearly straight. 
Dorsal valve of a medium convexity, most elevated about the middle ; the outline forming a 
uniform arch from the depressed beak to the front margin ; the slope from the umbo towards 
the cardinal angles, gently concave ; sometimes a barely perceptible mesial depression, com- 
mencing in a point at the beak, and becoming obsolete at one-half or two-thirds the length ; 
area small, lying in the plane of the lateral margins; peak minute, forming a small triangular 
projection, rising scarcely one-fourth of a line above the edge of the area, the point not incurved 
over, but situated in the plane of the area. Ventral valve moderately convex, most elevated 
at between one-fourth and one-third the length from the beak, thence descending with a some. 
what flat or gently convex slope, to the front and sides, and with a more sudden and somewhat 
concave siope to the hinge line and cardinal angles; the umbo small, prominent, neatly 
defined, terminating in a small rounded beak, which is incurved so as to overhang the e ige of 
the area, either not at all or scarcely one-tenth of a line ; area triangular, about one f urth 
larger than that of the dorsal valve, forming an angle of about 105° with the plane of the 
lateral margin. The foramen not observed, but appears to be wider than high. On looking 
at the dorsal valve in a direction perpendicular to the plane of the shell, the small rounded 
umbo of the ventral valve can be seen rising about one-third of a line above the dorsal beak. 
** Surface with small sub-angular radiating ridges, of nearly a uniform size, from eight 
to ten in the width of three lines, increasing by bifurcation, strongly curved outwards to the 
upper part of the sides and cardinal angles, the intervening grooves sub-angular in the bottom, 
and equal to the ridges in width. In very perfect specimens, very fine concentric sub-la- 
mellar concentric striz are visible, seven or eight to one line. In certain conditions of preser- 
vation also, the radiating ridges are seen to be sub-tubular, and exhibit numerous small ova! 
or circular openings on their edges, each about the eighth or tenth of a line in width, and 
from one-fourth to two-thirds of a line distant from each other. 
‘Width of largest specimen examined, eighteen lines ; length, sixteen lines; thickness 
or depth of both valves, seven lines ; height of area of ventral valve at the beak, one line; area of’ 
dorsal valve four-fifths of a line ; distance between the beaks, one line; length of hinge-line, ten 
lines. The most common size appears to be one inch in width. The beak of the ventral valve 
is incurved, so that it would touch a plane projected horizontally through the valve, at one-half 
the depth of the cavity. 
‘‘In some specimens the ventral valve has a faint, barely perceptible mesial fold, extend- 
ing from the umbo towards the front.” (Billings). 7 
I have very little to add to the above exhaustive description of O. Livia, by Mr. Bil- 
lings, the only two additional points of importance that I have observed being the follow- 
ing:—1. The foramen is very large, and is more than twice as wide at the base as it is high. 
2. The margin of the shell exiibits, in adult specimens, several well-marked concentric ridze# 
of growth, which are quite independent of the fine concentric strive which some examples present. 
