io86 The American Naturalist. [December, 
marks apply not to Orthoceras alone, but to Monticulipora, to 
Orthis, and to other large genera of early geologic time. 
One other genus will be referred to here. This is Fenestella. 
Professor Hall has probably studied this group as thoroughly as 
any one in this country. In his report as State Geologist of New 
York for 1882, he discusses the different genera which have been 
at times proposed for Fenestelloid forms of polyzoa ; and, after 
quoting the descriptions of thirteen of these, he remarks (p. 8) 
that " after an examination of hundreds of specimens, offering a 
wide range of variation, I am convinced that the genera mentioned 
above have only a sub-generic value, and should only be included 
in the comprehensive genus Fenestella." He then proceeds to show 
by a series of illustrations the failure of all the characters upon 
which the genera had been founded. He concludes no generic 
character can be drawn from the the celluliferous character of 
the dissepiments ; that the anastomosing of the branches is too 
indefinite to be a valid character ; that the number of ranges of 
all apertures is too inconstant, for often in this regard the 
features of several genera are found in one example ; and, finally, 
that the presence of a ridge or keel upon the branches is in many 
cases not even a good specific character. He then formulates a 
generic description of Fenestella broad enough to include the 
various forms. 
In a later publication (Report of the State Geologist of New 
York for 1884, Professor Hall returns to this subject, and gives 
short descriptions of seventeen sub-genera, recognizing the fact, 
however, that the boundaries of many of these are very indefinite, 
and that several may eventually have to be united under one. 
Fenestella is mainly an Upper Silurian genus, and it is evidently 
in the same formative condition as Monticulipora in the Lower 
Silurian. 
Does it not seem probable, then, that the disappearance from a 
certain horizen of a genus previously abundant, is the result of 
the differentiation of characters? If, for example, the very 
abundant genus Monticulipora of the Cincinnati rocks is present 
in greatly diminished numbers in the Niagara period, but is rep- 
resented there by numerous species of closely allied genera, it 
