50 RUGOSA. 



primary septa thus assume gradually the shape of unsymmetrically 

 dichotomous branches.* 



The following notation may render the idea more plainly. 



The primary septa can be represented thus : — 



s, s, 



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 : — 



s, Sl B. 



So So 



And further : — 



Si 



s, s, 



s 2 So 



s 3 s 3 



Sx Sx 



So 



s., 



Sx 



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

 we would have, after a few more steps : — 



s 2 s ' Sl Sl s 2 



S3 s 3 



s 4 s 4 



s 6 s 6 



S 10 S, 



s, s, 



So So 



S3 S3 



s 4 s 4 



S 6 . Sc 



S "> Sx s io 



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

 the Medusas ; for instance, Willia, Proboscidactyla. See Agassiz, Contr. Nat. Hist. U. S., Vol. IV. ; 

 and A. Agassiz, Til. Cat. Mus. Comp. Zool., No. II. 



