112 NOMENCLATURE. [CH. 



species in brackets if he was not the first to use the generic as 

 well as the specific name. Onychiopsis Mantelli (Brongn.) tells 

 us that Brongniart founded the species, but made use of some 

 other generic name than that which is now accepted. This 

 leads us to another point of some importance. Brongniart 

 described this characteristic Wealden fern under the name 

 Sphenopteris Mantelli; Sphenopteris being one of those ex- 

 tremely useful provisional generic terms which are used 

 in cases where we have no satisfactory proof of precise 

 botanical affinity. Sphenopteris stands for fern fronds having 

 a certain habit, form of segment and venation, and in this wide 

 sense it necessarily includes representatives of various divisions 

 and genera of Filices. If an example of a sphenopteroid frond 

 is discovered with sori or spores sufficiently well preserved to 

 enable us to determine its botanical position within narrower 

 limits, we may with advantage employ another genus in place 

 of the purely artificial form-genus which was originally chosen 

 as a consequence of imperfect knowledge. Fronds of this 

 Wealden fern have recently been found with well defined fertile 

 segments having a form apparently identical with that which 

 characterises the polypodiaceous genus Onychium. For this 

 reason the name Onychiopsis has been adopted. It is safer and 

 more convenient to use a name which differs in its termination 

 from that of the recent plant with which we believe the fossil 

 to be closely related. A common custom is to slightly alter the 

 recent name by adding the termination -opsis or -ites. There 

 are several other provisional generic terms that are often used 

 in Fossil Botany, and which might be advantageously chosen 

 in many cases where the misleading resemblance of external 

 form has often given rise to the use of a name implying 

 affinities which cannot be satisfactorily demonstrated. 



It was the custom of some of the earliest writers, in spite of 

 their habit of using the names of recent Flowering plants for 

 extinct Palaeozoic species of Vascular Cryptogams, to adopt 

 also general and comprehensive terms. We find such a name 

 as Lithoxylon employed by Lhwyd^ in 1699 as a convenient 

 designation for fossil wood. 



1 Lhwyd (1699). 



