78 Rev. N. Glass—The Spirals of the Brachiopoda. 
and their connections with each other, but only the first discovered 
example will be given of each distinct and peculiar form. The 
names and dates given in brackets refer in each case simply to the 
discovery of the calcareous spirals. 
Tue Positron oF THE SPIRALS IN THE SHELL. 
In the Spiriferide the spirals have their bases facing each other 
in the centre of the shell, but usually their apices have a more or 
less upward direction towards the posterior angle of the lateral 
margins of the shell. This upward direction is very marked in 
Spirifera lineata, var. imbricata. The spirals are sometimes directed 
backwards into the rostral cavity of the ventral valve as well as 
upwards, for example in Oyrtina heteroclita. 
In all the other families or groups of the spiral-bearing Brachio- 
poda the spirals are always so disposed as that a transverse section 
taken through the greatest circumference of the shell would as 
nearly as possible pass through the apices of the two spirals and the 
centre of their respective bases. The spirals which are thus 
arranged in the shell have, so far as has been ascertained, six 
different positions in the various genera to which they belong. 
Supposing that the dorsal or lesser valve in a specimen of Athyris 
has been removed, the spirals will then appear with their bases 
facing each other in the centre of the shell, and their apices directed 
towards the lateral margins of the shell. The five remaining 
positions of the spirals referred to might all be obtained by rotating 
the spirals simultaneously each on its own axis—that is, its perpen- 
dicular axis, from the posterior to the anterior border of the spiral. 
(Of course this could not be done in the specimen itself, but the 
motion might be illustrated by two cones cut out in wood.) Thus, 
in the case of Athyris as described above, if the two spirals were 
rotated simultaneously outwards each on its own perpendicular axis, 
that is, the right-hand spiral to the right and the left-hand spiral to 
the left, the spirals would soon assume the position in the shell and 
towards each other of the genus Dayia, Dav. (Glass, 1880), in which 
the apices of the spirals face the middle of the lateral portions of 
the ventral valve. Continuing the rotation Thecospira (Zugmayer, 
1880) is reached, in which the apices of the spirals face the bottom 
of the ventral valve. Again continuing the rotation we have succes- 
sively Glassia, Davidson, 1881, in which the apices of the spirals face 
each other in the centre of the shell, Zygospira, Hall (Whitfield, 
1862), in which the apices of the spirals are directed obliquely into 
the cavity of the dorsal valve, and Aétrypa, in which the apices 
of the spirals face the bottom of the dorsal valve. And a still 
further rotation would bring the apices of the spirals to their first 
position as in Athyris. 
Tue ATTACHMENTS OF THE SPIRALS TO THE HINGE-PLATE OF THE 
Dorsat VALVE. 
There is not much variety here. In the Spiriferide the attach- 
ments are straight. In the Nucleospiride and Athyride, and in 
