AGLAOPHENIA SIGMA. 45 



nary form for some distance from its origin. It is, however, somewhat 

 more attenuated, while its hydrothecaj are slightly smaller than in the 

 ordinary pinna3. In the specimens examined these hydrotheca? 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 pinna suddenly commences and continues to its distal end. 



This transformation consists in the hydrothecae ceasing to be adnate 

 to the racliis of the pinnae, 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 (b) retain nearly their ordinary shape and size, but the mesial 

 nematophore (c) 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 pinna, which remains nearly imaltered, 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 pinnae 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 corbuLi. 



The joints, which are very distinct in the proximal portion of the 

 pinna, become obsolete in the corbula. 



Aglaophenia sigma. 



Fl. XXVI. Figs. 9, 10. 



Trophosome. — Hydrocaulus attaining a height of about six('?) inches, 

 simple, fascicled almost to the tip; pinna? alternate, with their origin 

 from the front aspect of the stem. Hydrothecae deep, nearly cylindrical, 



