Chap. H. GENERIC AND SPECIFIC CHARACTERS. 81 



cavity, but extends to the extremity of the so-called arms. The peculiar lobed 

 outline of the disk is owing to the develoiDment of the sj^stem of radiating tubes; 

 and the evidence of this connection may be found in the fact, that the deeper 

 emarginations correspond to the position of the eyes, at the end of eight simple, 

 radiating tubes, and the lesser emarginations to the ends of similar simple tubes 

 without ej'es, combined with an even development of comparatively small tentacles, 

 along the whole margin, with the exception of the spaces occupied by the eyes, 

 which are, however, themselves modified tentacles. It is, therefore, plain that the 

 form of Aurelia presents a pattern distinct from that of Cyanea, in which the ten- 

 tacles are gathered up in large Inmches, on the under surface of the disk, at con- 

 siderable distance from the margin, facing deep indentations of its outline, much 

 deeper, indeed, than those of the Aurelia, and occupying a position homological to 

 that of the lesser indentations of the latter. It differs equally, though in a different 

 way, from Sthenonia, in which the position and arrangement of the tentacles recall 

 Cyanea, Avhile the lobes of the margin are different from both, and the oral append- 

 ages quite diminutive. We shall have an opportunity, hereafter, to show that Pelagia 

 must be considered as the type of another family. 



SECTION IX. 



GENERIC CHARACTERS OF AURELIA, AXD SPECIFIC CHARACTERS OF THE AURELIA FLAVIDULA 



OF NORTH AMERICA. 



In f\xmilies composed of a single genus, naturalists have generally been satisfied 

 Avith the statement, that the generic character coincides with that of the family ; 

 but, if genera are founded in nature and based upon a different category of char- 

 acters from those which distinguish families, this practice ought not to prevail. It 

 may be more difficult to ascertain the characteristics of a genus which stands 

 alone, and to discriminate between those structural features which are generic and 

 those which belong to the family ; but, surely, if a second genus should be dis- 

 covered at a later time, belonging to a family up to that period containing a 

 single genus, from that time forward, at least, the older genus could no longer 

 be said to be characterized by the same features as the family. Our ignorance, 

 therefore, of the existence or non-existence of other genera in nature does not 

 alter the case, and I hold that it is incumbent upon a naturalist, at least to attempt 

 to trace the characters of such a genus. In the family of Aurelidaj, it appears 

 to me, that the single genus of which I have any knowledge is likely to be 

 characterized by those structural peculiarities which, having no direct bearing upon 



VOL. IV. 11 



