48 DTOONITES. 



Type. Imperfect fragment of frond. 



In 1833 Mantell described a badly preserved frond fragment 

 from the Tilgate beds of Sussex, which he named Cycadites Brong- 

 niarti, using the term Cycadites rather as a general designation 

 indicative of cycadean affinity, and not in accordance with the 

 narrow sense in which Brongniart defined the genus. Morris sub- 

 stituted Pterophyttum for Cycadites, and Dunker called attention 

 to the plant figured by Homer as C. Brongniarti, which should not 

 be confounded with Mantell' s type described under the same name ; 

 the latter he suggested should be referred to Nilssonia. Goppert 

 adopts Nilssonia as the generic term, and Miquel, Ettingshausen, 

 and others follow his example. Schenk, on the other hand, 

 points out certain discrepancies between the characters of Mantell' s 

 species, as further illustrated by subsequently described examples 

 of the same type from the Wealden of North Germany, and the 

 genus Nilssonia ; he substitutes Miquel's term Dioonites for 

 Bunker's Nilssonia. 1 Schenk's specimen is in a better state of 

 preservation than the English example, and shows more clearly 

 the manner of attachment of the pinnae. This species differs from 

 Otozamites Gbppertianus (Dunk.), in the absence of an auriculate 

 base to the segments, in its coarser veins, and in the segments 

 being more nearly at right angles to the axis of the frond. We 

 may adopt Schenk's definition : " Folia pinnata, segmenta e basi 

 latiore apicem versus attenuata acuminata lineari-lanceolata integra 

 approximata alterna vel opposita, in petioli latere antico sessilia, 

 3 mm. usque ad 2'5 cm. longa, 3'5 5 mm. lata, superiora breviora, 

 summa brevissima ovata, superiora oblique patentia, media paten- 

 tissima, nervi tenues quinque vel sex tenues aequales paralleli." 



The plant figured by Leckenby 2 as Pterophyttum angustifolinm, 

 Leek., from the Oolite of Gristhorpe, shows a marked similarity 

 in general appearance to Dioonites Brongniarti. 



Yokoyama's Japanese species, D. Jfotoei, 3 may probably be 

 included as a synonym of Mantell's plant ; the former author 

 mentions the greater number of veins in the pinnae of his plant 

 as a distinguishing feature from D. Brongniarti, and speaks of the 



1 Schenk (A. 2), p. 34. 



2 (A.), pi. riii. fig. 3. 



3 Yokoyaraa (A. 2), p. 44. 



