CHAPTER XLI. 



GENERA BELIEVED TO BELONG TO THE GINKGOALES BUT 

 WHICH ON THE AVAILABLE EVIDENCE CANNOT BE RE- 

 FERRED WITHOUT HESITATION TO THAT GROUP. 



GINKGODIUM. Yokoyama. 



Yokoyama^ defined the genus as follows: 'Leaf coriaceous, 

 entire or lobed, gradually narrowed towards the base which is 

 thickened at its margin and gradually passes into a short petiole ; 

 veins numerous, simple, parallel; interstitial veins very fine.' 

 He draws attention to the thickening of the lower margin of the 

 lamina, a feature reminiscent of Ginkgo and to the course of the 

 veins which run parallel to the median axis of the lamina instead 

 of spreading from the base as in Ginkgoites and Baiera. Gink- 

 godium resembles the Palaeozoic genus Whittleseya in the position 

 of the veins but the genera are unlikely to be confused; a com- 

 parison may also be made with the Jurassic genus EretTnophyllum 

 (fig. 658) which has longer and narrower leaves with a coarser 

 venation. We have no information with regard to the cuticular 

 structure, the nature of the supporting axes or reproductive organs. 

 The supposed affinity to Ginkgo rests therefore on leaf-form alone. 



Ginkgodium Nathorsti Yokoyama. 



The type-species was founded on specimens from strata in 

 Japan assigned by Yokoyama^ to the Middle Jurassic series, but 

 the flora suggests a somewhat higher horizon in the Jurassic 

 system. Some of the leaves are entire, obovate, and have a 

 truncate distal end ; others are cuneate and broader at the apex 

 which may be lobed, while in some forms the leaf is divided by a 



1 Yokoyama (89) B. p. 56. 



- Ihid. p. 57, Pis. n., m., vra., ix., xn. 



