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correctness of this assertion the author selects 8 species of the 
128 treated in his work, in which the characters taken from the 
margin and operculum either are not found in some species. of 
a certain genus as defined by him or in some species of two 
other genera show 'a supposed variation or inconstancy, and as- a 
consequence hereof' he prefers jto use as chief characters in the 
limitation of most genéra .diversities in the årrangement of the 
hydrothecae or in the form of the colony, but at the same time 
he lays the chief stress on the opercular apparatus and, the form 
of the hydrotheca in the diagnosis of the two genera Diphasia and 
Abietinaria. 1 intend here to undertake a critical examination of 
the genera proposed by Prof. Nutting, and under each genus I 
shall mention not only the species in which this author has thought 
he found inconstancy in. the hydrothecal characters, but also those 
which to my opinion ought to be referred to another genus. — 
In his work on; the American Sertulariidae Prof. Nutting 
gives the following diagnosis of the genera as far as concerns 
the trophosomes, but as the gonosomes on the whole play åa very 
insignificant systematic part, I have not found it necessary to refer 
to that part of the diagnosis concerning these structures. I shall 
later mention these structures in my own diagnoses of the same 
genera. 
"Sertularia (L.) Nutting. 
"Hydrothecæ in strictly opposite or rarely subopposite pairs. 
Stem and branches normally divided into regular internodes, each 
of which bears a pair of hydrothecæ, but sometimes there are more 
" than one pair to the internode, in which case the hydrothecæ are 
strictly opposite. Operculum normally of two flaps.” 
Thujaria (Flem.) Nutting. 
«"Hydrothecæ normally subopposite to alternate, and more than 
two to each internode. " Internodes vary greatly in length. Hydro- 
thecæ with smooth margin, or with one or two teeth usually more 
or less immersed in the- hydrocaulus. Operculum of one abcauline 
flap, or of two flaps." 
