XL] 



GINKGOITES 



15 



fig. 633). The number of Ginkgoites leaves from Jurassic strata is 

 considerable and the student who attempts to classify specimens in 

 a large collection under specific heads soon finds himself confronted 

 in an acute form with the constantly recurring difficulty of fixing 

 boundaries. As Knowlton^ says, 'In dealing with such an abun- 

 dance of specimens and multiplicity of forms one must needs make 

 either many " species" to accommodate this diversity, or only one 

 or two, and in view of the known variation exhibited by the single 

 living species, the latter plan seems preferable.' In advocating this 

 use of specific names in a liberal sense I admit the probability or in- 

 deed the certainty that forms specifically distinct will be grouped 



Fig. 633. Ginkgoites digiiata var. Huttoni. A leaf from the Upper Jurassic of 

 Helmsdale, Scotland. (Stockholm Museum ; nat. size.) 



under one designation. It is, however, clearly impossible in the 

 case of impressions of leaves of Ginkgoites to impose limitations 

 based on the form of the lamina, the degree of dissection, and similar 

 variable features that cannot be accepted as trustworthy criteria of 

 true specific distinctions. As material accumulates data may be 

 furnished that will enable us to recognise characters of morpho- 

 logical significance : in carbonised impressions from which cuticular 

 preparations can be made the form of the epidermal cells and 

 the structure of the stomata may supply a valuable aid to more 

 accurate diagnosis. The spacing of the veins is a feature worthy 

 of attention in the description of well-preserved specimens. 

 1 KnowHon (14) p. 55. 



