34 
The dorsal valve shows a double or bilobed cardinal process, with the strong crural bases 
supporting spiral arms which are directed into the two compartments of the ventral valve, and 
making numerous turns, terminate in the rostral part of the shell.” (Hall, Pal. New York, 
Vol. IV., p. 268). . 
I have little to add to the above detailed description of this pretty little Brachiopod by 
Professor Hall. Cyrtina Hamiltonensis occurs commonly, and in a state of beautiful preser- 
vation, in the Hamilton Formation of Ontario, and I have also seen casts apparently belong- 
ing to this species from the Corniferous Limestone. Many of the Hamilton specimens have 
a flat area without an incurved beak, and thus come to agree with Hall’s Cyrtina Hamilton- 
ensis var. recta ; but I am of opinion that this variety is by no means sufficiently distinct to 
deserve a separate designation. In fact there appear to be three chief variations as regards 
the above mentioned points within the limits of this mutable species, and all three are con- 
nected together by transitional forms. In one of these, the area is nearly or quite plane, and 
is directed forwards almost or quite at right angles to the plane of the dorsal valve or hinge- 
line, whilst the beak is very slightly elevated, and little or not at all incurved. In these forms 
the thickness of the shell (measured at right angles to the hinge-line, instead of from the hinge- 
line to the beak) is proportionately very great. In other forms, the hinge-area is moderately 
elevated and forms a very obtuse angle with the plane of the dorsal valve, whilst the beak is 
moderately prominent and is incurved. These forms may be regarded as being the normal 
form of the species. In other examples, again, which are otherwise undistinguishable from 
the preceding, the beak of the ventral valve is extraordinarily elevated and prominent, and 
is markedly incurved ; whilst the plane of the arcuate area approximates to the plane of the 
dorsal valve and forms with it an acute angle. The result of this is that the thickness of 
the shell measured at right angles to the hinge-line becomes very much smaller than in the 
preceding forms. The last mentioned form.appears at first sight to be a distinct species from 
those with a flat area directed perpendicularly to the plane of the dorsal valve, but I am 
satisfied that all the above mentioned varieties may be safely brought under the limits of a 
single species. 
The dimensions of this species vary considerably. In an adult example of the variety in 
which the area is flat and perpendicular to the plane of the dorsal valve, the width of the 
hinge-line is ten lines, the length of the dorsal valve is five lines, the length of the ventral valve 
is eight lines, the height of the area is five lines, and the thickness of the shell measured at 
right angles to the hinge-line is also five lines, and is thus equal to the height of the area, In 
another also fully-grown example, with a very oblique area, the width of the hinge-line is 
nine lines (the cardinal angles being much more rounded than in the preceding), the length of 
the dorsal valve is six lines, the length of the ventral valve is ten lines, the height of the much 
curved area is nearly four lines, and the thickness of the shell at right angles to the hinge-line 
is only three lines and a half, and is thus much less as compared with the size of the shell than 
in the preceding. 
Small examples as well as large are not uncommon, showing the variations in the plane 
of the area here described, proving that the latter are not due to variations of age. 
Locality and Formation.—Common in the Hamilton Shales of Widder, in the Township 
of Bosanquet. 
Genus SPIRIGERA (D’Orbigny). 
Shell variable in form, with convex valves which are articulated by teeth and sockets ; 
beak of the ventral valve short and incurved, and truncated by a small foramen, which is in 
contact with the beak of the dorsal valve, or is separated from it by a deltidium of two pieces ; 
a mesial fold and sinus present or absent; surface smooth or striated, with numerous con- 
centric lines of growth; dorsal valve without any mesial septum, but with spiral cones, the 
extremities of which are directed outwards towards the lateral margins of the shell. 
It seems unnecessary to enter here into the controversy which has been carried on as to 
the genera Spirigera, Athyris, Merista, and Meristella, or to attempt to decide which of those 
divisions can be retained, and which must be rejected. It is sufficient to state that I shall 
here adopt the arrangement proposed by Mr. Davidson, and followed by Mr. Billings, in which 
Spirigera is made to include shells without a mesial septum and with the beak of the ventral 
valve perforated by a conspicuous foramen, whilst Athyris includes shells with an imperforate 
beak (in the adult state at any rate), and generally with a mesial septum. Adopting this 
