Jurassic plants from Yorkshire, ete. 257 
including the petiole which measures from 1—3 cms. The margins 
of the lamina are entire and the apex either bluntly rounded or 
somewhat retuse (cf. fig. 2), in the latter case the apical notch is 
obliquely placed. Towards the base of the lamina the margins 
are frequently thickened on one face, the thickened portions 
passing into the petiole in the way usually seen in the leaves of 
the recent Ginkgo biloba. The petiole is somewhat expanded at 
its base. 
The venation is usually very distinct. Two or three veins 
coming up from the petiole become distinguishable at the base 
of the lamina and soon dichotomise several times, the resulting 
divisions run parallel to each other through most of the length 
of the leaf (fig. 1), though in the more obovate forms they may 
again fork occasionally. Near the apex the veins converge some- 
what. The veins are large and widely separated ; they are from 
1mm. to 15 mm. apart. Ina leaf of average size 13 veins occur 
in a width of 16mm. Some of the leaves are now in a peculiar 
brown translucent condition and their veins can be often seen 
under the microscope to consist of a number of fine parallel dark 
strands, no doubt representing the original xylem or sclerenchyma 
strands. 
Between the veins in some of the larger specimens a row of 
small spots or short lines may be observed which are visible to 
the naked eye and are 2—3 mm. apart. These are the secretory 
tracts which are seen in the corresponding position in recent 
Ginkgo leaves, which have been figured in the Jurassic Ginkgo 
Obrutschewi by Prof. Seward* and which I found to be present 
in some Ginkgo leaves from the Yorkshire Jurassic collected by 
Prof. Nathorst and now in the Stockholm Museum. When the 
leaves are macerated, these little secretory tracts yield small 
groups of cell-like structures which resist the action of the acid 
and might in some circumstances be taken for groups of spores. 
The Cuticular Structure. 
As is usually the case with stout leaves that separate readily 
from the matrix, the present specimens yield excellent cuticular 
preparations in which the outlines of the epidermal cells and the 
stomatal openings are clearly seen. In H. pubescens the upper 
and lower cuticles are very distinct. The upper epidermis was 
composed of more or less uniform cells and was devoid of stomata. 
The cells were irregularly angular or polygonal in shape, becoming 
somewhat elongated above the veins. Their walls, which are 
usually straight, sometimes show the slight undulations of the 
* Seward (11), p. 46, Pl. rv. figs. 42, 43. 
