AGLAOPHENIA SIGMA. 45 
nary form for some distance from its origin. It is, however, somewhat 
more attenuated, while its hydrothecw are slightly smaller than in the 
ordinary pinne. In the specimens examined these hydrothece were three 
in number, and the first and third internode carried each a small accessory 
mesial nematophore (not represented in the figure) at the proximal side of 
the principal one. After the third internode the principal transformation 
of the pmna suddenly commences and continues to its distal end. 
This transformation consists in the hydrothece ceasing to be adnate 
to the rachis of the pinne, and becoming elevated on short stalks while 
they become at the same time approximated and thrown alternately to 
the right and left, so that the pinna carries now two alternate rows 
of short processes, each bearing a little cup similar to that of the ordi- 
nary hydrotheca except in being somewhat smaller. 
With the elevation of the hydrotheca above the level of the rachis 
the supracalycine and mesial nematophores are carried up with it (Fig. 8). 
The former (4) retaim nearly their ordinary shape and size, but the mesial 
nematophore (¢) becomes enormously developed, being not only greatly 
increased in length, but becoming broad, flattened, and somewhat sabre- 
shaped, while a row of small tubular nematophores is developed along 
the distal edge of each, as well as along the proximal edge of the stalks 
(a) which carry them. 
It is the mesial nematophores thus singularly transformed which 
mainly constitute the ribs which form the sides of the corbula. Be- 
tween these and the mesial nematophores of the proximal portion of 
the pimna, which remains nearly unaltered, there is no gradual transition, 
but it is interesting to note that the internodes of this part of the 
pinna differ from those of the ordinary pinne in carrying a small acces- 
sory mesial nematophore, which is repeated and multiplied on the short 
stalks which form the bases of the ribs in the corbula. 
The jomts, which are very distinct in the proximal portion of the 
pinna, become obsolete in the corbula. 
Aglaophenia sigma. 
PU EXVA> Bigs 9s, 10! 
Trophosome. — Hydrocaulus attaining a height of about six(?) inches, 
simple, fascicled almost to the tip; pinne alternate, with their origin 
from the front aspect of the stem. Hydrothece deep, nearly cylindrical, 
