81 
= — 
developed, is only from half a line to three quarters of a line, and is often less. _Commenc- 
ing at the beak and extending to the front of the ventral valve is a well-defined mesial sinus, 
which is usually shallow and rounded, and is bounded laterally by plications stronger than 
the rest. 
Dorsal valve moderately convex, not so convex as the ventral valve, its beak incurved, 
but not in contact with that of the ventral valve, its area extremely narrow and linear. Cor- 
responding with the sinus of the ventral valve, there is a prominent mesial fold extending 
from the beak to the front margin. 
Surface marked by sub-angular radiating ribs or plications, usually from eight to twelve 
on each side of mesial fold and sinus, the outermost not reaching the beak, and the cardinal 
angles being free from them. The plications are crossed by numerous imbricating zig-zag 
concentric striae, which become very much crowded together as the margin of the shell is 
approached, and which, close to the margin, are often interrupted by stronger lines of growth. 
The dimensions and form of Spirifera mucronata are excessively variable—so much so 
that the extreme terms of the series have all the appearance of being distinct species. The 
following gives the form and measurements of the leading varieties of this species as seen in 
the Hamilton Formation of Ontario, where it is a wonderfully abundant species, and is found 
in a state of beautiful preservation :— 
a. A semi-circular, and very gibbous form, with the cardinal angles acute, but hardly 
produced, width of hinge-line one inch; length eight lines; height seven lines. 
b. A moderately convex form, with the cardinal angles moderately but not excessively 
produced ; the general figure of the shell becoming trigonal. Width at hinge-line one inch and 
seven lines; length, eleven lines; thickness, seven lines, This may be regarded as the normal 
form of the species. 
c. A form upon the whole resembling the preceding in outline, but with straighter sides, 
so that the shell is of a regular triangular shape, the apex of the triangle being truncated at the 
mesial sinus and fold. The shell is also much thinner owing to the smaller convexity of the 
valves, and especially of the dorsal valve, which is gently arched near the umbo, but is more 
or less abruptly reflected all round the margin at about two-thirds of its length from the beak. 
The cardinal angles are also more extended, increasing the disproportion between the width 
and length. Width at hinge-line, twenty-two lines; length, nine lines; height, only four lines. 
This form occurs not uncommonly in the railway-cutting at Widder, and can be traced by 
insensible gradations into the preceding, just as that graduates into the form first described. 
d. Lastly, a form in which the cardinal angles of the shell are immensely produced, the 
total width at the hinge-line thus coming to be three, four, five, or even six times as great as 
the length at the beaks. There are, also, from fifteen to twenty plications on either side of the 
mesial fold and sinus. Width at hinge-line in a small specimen twenty-two lines; length, 
seven lines; height, three lines and a quarter. Width at hinge-line in a fully-grown specimen 
three inches; length, six lines; height apparently between four and five lines. 
Locality and Formation.—Very abundant and beautifully preserved in the decomposed 
shales of the Hamilton group, on the line of the Grand Trunk Railway, near Widder Station, 
where only the first three of the varieties here described are known to occur. Also, common 
in the calcareous or caleareo-arenaceous beds of the same formation at Bartlett’s Mills, near 
Arkona, in the Township of Bosanquet, where the extremely mucronate variety is the chief, 
or only, one to occur. Also, in the Corniferous Limestone of Port Colborne and Ridgeway. 
97. SPIRIFERA VARICOSA (Hall). 
Spirifer varicosus (Hall), Tenth Report on the State Cabinet, p. 130. 
Spirifera varicosa (Billings), Canadian Journal, New Series, Vol. VI., p. 255, Figs. 
63, 64. 
Spirtfera varicosa (Hall), Pal. New York, Vol. IV., Plate XXXI., Figs. 1-4. 
This species is very closely allied to Spirifera mucronata, with which it agrees in its general 
form and its surface characters. The chief point by which S. varicosais distinguished is the 
comparatively great height of the ventral area, and the prominence of the beak of the ventral 
valve. Thus in S. mucronata, the ventral area rarely exceeds half a line in height, whilst in S. 
varicosa it may have a height of as much as two lines and a half. The latter species is also in 
6 
