172 THE LATE DR. NEWELL ARBER : CRITICAL STUDIES 
These remarks apply in particular to those cones which have comparatively 
undifferentiated sporophylis, the standard of comparison here as regards 
differentiation being the leaf. In dealing with impressions of such cones it 
is, of course, quite impossible to recognise those characters which in the case 
of petrifactions are regarded as of taxonomic value. Such, in fact, are only 
visible under the microscope. For these reasons the classification of im- 
pressions of Lepidostrobi has always been a difficult matter, and authorities 
have differed as to the species which should be recognised, and as to their 
essential characters. 
A desire for more definite progress in this respect has, indeed, been felt 
from quite early times. As far back as 1838, Brongniart (3. ii. pl. 12) 
ilustrated the range of sporophyll-form met with among the living 
Lycopods, to serve as a basis for such studies, and the analogy has also been 
pointed out by others since. I do not, however, propose to enter further into 
previous work on this subject here, for it will be necessary frequently to 
refer to the chief authorities in the course of this revision of the genus. 
In attempting here to reclassify the British Lepidostrobi occurring as 
incrustations in the Coal Measures, I take as my specific unit the shape of 
the sporophyll. 
I am unable to find any critical characters, whether derived from the 
features of the axis of the cone, or from any other details of its organisation, 
which ure at all comparable in taxonomic value to the sporophyll-form. 
The sporophylls of these cones have naturally two regions, the proximal 
and the distal. The proximal portion, often egjled the pedicel, is the lower 
part of the sporophyll, including the attachment to the axis and the surface 
bearing the sporangium. The position of the ligule marks the distal 
extremity of this region, and this rtidimentary organ, of course, indicates 
that morphologically this portion of the sporophyll is an expanded leaf base. 
Very often part of the pedicel, especially that subtending the sporangium, is 
modified by the formation of a cushion or other enlarged growth, on which 
the sporangium is seated or to which it is attached. The distal portion of 
the sporophyll, often termed the lamina, extending from the region of the 
ligule to the apex of the sporophyll, is as a rule less modified, 2. e., more leaf- 
like, than the pedicel. ~ Yet modifications sometimes occur here also, as, for 
instance, the development of a downwardly directed heel in the region of the 
ligule. l 
In most impressions of Lepidostrobi, it is usually possible to make out the 
essential features of both regions of the sporophyll, and it is on these 
characters that the species should be founded. Whether the cone is homo- 
sporous or heterosporous does not affect the question, for, as we know, the 
sporophylls subtending the micro- and megasporangia of the same cone are 
alike, and often precisely similar to those of homosporous cones. 
From the study of considerable material, consisting of various cones of 
the Lepidostrobus type, preserved in the Sedgwick Museum, Cambridge, 
