38 DIOONITES. 



On the whole, then, the characters generally insisted on seem to 

 be the lateral attachment of the pinnae to the rachis, and hy 

 many, but by no means all authors, the truncately terminated 

 segments. The confusion which has arisen from constant altera- 

 tions by various writers, and from the not uncommon practice 

 of including certain fronds in a particular genus, in spite of 

 obvious discrepancies between the specimens and the generic 

 diagnosis, is sufficiently obvious if we glance at some of the 

 better known Pterophyllum species as figured by different authors. 

 We have such forms as P. inconstans, Gopp., P. Dunkerianum, 

 Gb'pp., P. Jaegeri, Brong., P. Braunii, Gopp., etc., included in the 

 same genus. It is true that in examining fossil fronds we are 

 often unable to decide as to the actual manner of attachment of 

 the pinnae, and are thus driven to leave the specimen as doubtful, 

 or to decide as best we may in the face of difficulties inseparable 

 from the determination of isolated leaf fragments. We cannot 

 always be sure whether we have the frond preserved with its 

 lower or upper side uppermost. It is, however, clear that we 

 cannot consistently make use of Brongniart's genus for such 

 specimens as those before us. 



The genus Dioonites of Miquel has been adopted by some 

 authors for these narrowly segmented Wealden fronds. This 

 again is a generic name which has been made to do duty for 

 forms of leaves, which it is difficult to regard as correctly included 

 in the same genus, even if the genus be admittedly a provisional 

 and artificial one. Miquel is responsible for the proposal of this 

 name, and for the following definition: 1 "Frondes pinnatae, 

 rigidae, crassae. Foliola densa patentissima suprema nunc sub- 

 irnbricata, lanceolata, vel lineari-lanceolata, recta vel subfalcata, 

 acuta vel acutiuscula, basi tota latitudine inserta, inferne 

 retrorsum subdecurrentia, nervis cum margine parallelis aequa- 

 libus subtus distinctioribus (cum sulculis stomatiferis alter- 

 nantibus)." He included under this name several species 

 previously described as examples of Pterophyllum and other 

 genera. Bornernann adopts Miquel's genus and extends its use 

 to some additional species, but does not make any important 

 alteration in the original diagnosis. Schimper retains the term 



(2), p. 7. 



