Bd. III: 14) 
THE MESOZOIC FLORA. 
5 
this rule. If a certain form from a distant region resembles some other known form, 
but at the same time shows sufficient peculiarities of its own to be set aside as a 
variety, the probability is that the two forms belong to quite distinct species. There 
is, at any rate, no evidence that the one form stands in such genetic relation to 
the other as is implied by its designation as a variety of the latter. The ternary 
nomenclature which was used for similar cases, for instance, by Heer and was at 
one time advocated by Prof. Nathorst (1886), is no doubt better. Serious objec- 
tions must, however, be raised against a departure from the binary nomenclature 
that might set an example for other deviations of a less desirable nature. It is at 
any rate simpler to use the same specific name or to institute a new species — accor- 
ding to the degree of resemblance — , since the forms in question most probably are 
either specifically identical or quite distinct, and the palæobotanical species is in 
any case only artificial, as a rule. 
A special question is how to design sterile forms which appear identical with 
such fertile ones from another region which have been referred to a genus founded 
on the structure of the fructification. It has been customary, in such cases, to use 
the generic name created for the fertile form, even though only sterile specimens 
are present. This does not involve any great risk when the structure of the vegeta- 
tive parts found is very characteristic and peculiar, but in many cases leaves or other 
vegetative parts, which present sufficient agreement to be referred to one and the same 
form-species, may have had quite different fructifications, on account of which they 
should be referred not merely to different natural species but to different genera and 
families. This is an unevitable consequence of the fact that a form-species instituted 
for sterile fronds has no claim to be necessarily a species in a systematic sense, and 
the more widely the form-species are delimited, the greater must be the risk. Of 
the genus CladopJilebis^ for instance, several ot the previously used form-species are 
now referred, on account of the fructifications, to Toditcs Sew. and compared with 
the Osmundaceæ (Seward 1900, p. 86), C. I obi folia has recently been made the 
type of a new genus, Eboracia (Thomas 1911, p. 387), considered as a member of 
the Cyatheaceæ, whereas other forms belonging to the same form-genus show affini- 
ties to the Schizæaceæ. Yet, the different species of CladopJilcbis are by no means 
well defined or easily distinguishable from each other. In other cases the form- 
species may have a greater systematic value; but the safest way is, no doubt, to 
use the form-genera for sterile forms from a distant region. This rule has been 
followed here for the most part; but the name of the genus to which the probably 
corresponding fructification is referred has in such cases been given in brackets. 
