50 RUGOSA. 



primary septa thus assume gradually the shape of uusynuiietricallv 

 (licliotomous branches.* 



The following notation may render the idea more pliunl3^ 



The primary septa can be represented thus : — 



Si 



Si Si 



Si S, 



S, 



The upper and lower being in the longer axis, the lower S being the 

 one in the septal fossa. The next step of development will be : — 



And further : — 



S, 



Sa S3 



Si 



Still continuing with Milne-Edwards and Ilaime's system of notation, 

 we would have, after a few more stejis : — 



s., ^^ ^' ^i s, 



S3 S3 



s, s^ 



Sio ^10 



Si Si 



Sj Sj 



S3 S3 



S4 s< 



^>o Sj ''■o 



* A somewhat similar unsymmetrical development of the .systems has been observed in some of 

 the MedusiP ; for instance, Willia, Proboscidacti/la. See Agassiz, Contr. Nat. Hist. U. S., Vol. IV. ; 

 and A. Agassiz, III. Cat. Miis. Comp. Z06I., No. II. 



