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 : — 



Si 



Si Si 



Si Si 



Si 



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 : — 



Si Si 



Si Si 



And further : — 



S, 



Si ^^ Si 



S2 §2 



S3 S3 



Si Si 



S2 S2 



S3 _ S3 



01 



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

 we would have, after a few more steps : — 



Si Si Si 



^10 s^ Sio 



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

 the Medusa; ; for instance, Willla, Prohoscidactijla. See Agassiz, Contr. Nat. Hist. U. S., Vol. IV. ; 

 and A. Agassiz, 111. Cat. Mus. Comp. Zobl., No. II. 



