ONTOHIOPSIS. 57 



portions, sometimes extremely minute portions, of fronds, and all 

 placed in the recent genus Thyrsopteris. One would naturally 

 expect that this remarkable assemblage of species, called by the 

 name of a unique living genus, should rest on trustworthy 

 evidence. The author of these forty species himself admits that 

 no fructification has been found; surely this admission will be 

 sufficient in itself to make botanists pause before they allow them- 

 selves to be lost in contemplation of the wealth of specific form 

 displayed by these Potomac plants, or to assume as an established 

 fact that Kunze's solitary species was represented in the Potomac 

 vegetation by such a host of ancestors. We may justly ask — Are 

 there differences enough, and sufficiently well marked, between 

 these numerous varieties to warrant the result arrived at ; and, 

 secondly, is the evidence at hand strong enough to justify the use 

 of the name Thyrsopteris, or even a modification of it which 

 would be suggestive of something not quite so certain and well 

 founded ? 



To the first question I am strongly of opinion that the answer 

 is a decided negative. It must be admitted that my evidence is 

 entirely based on an examination of Fontaine's figures and descrip- 

 tions, and has not the weight of testimony derived from an actual 

 inspection of the specimens themselves. To the second question, 

 the admission that no trace of a fertile pinna or pinnule has been 

 recognized is, I am disposed to think, a sufficient reply. It would 

 be a presumption and beyond my province, to attempt to describe 

 how many species are represented by the " Thyrsopteris" specimens 

 found in the Potomac beds ; but it is at least possible to indicate a 

 few of those cases in which the determinations of Fontaine are not 

 such as I feel able to accept. 



The species T. Virginica ' is founded on specimens of the "pinnae 

 of ultimate order,'' and the fragments figured might easily be 

 included in some of the thirty-nine remaining forms without the 

 unnecessary institution of an additional species. Another instance 

 of the same kind is afforded by T. alata, which depends for its 

 existence on part of a pinna apparently indistinguishable from 

 T. Meehiana. 



In describing T. rarinervis ' Fontaine refers to it as " one of the 



' Lie. nt. p. 120, pi. xxiv. ] 

 * Loo. cit. p. 123. 



