92 Scott and Maslen . — The Structure of Trigonocarpus . 
overlaid by two or three layers of thin-walled cells, which in all 
probability represent but a small portion of a much more extensive tissue, 
so that, according to this character, Trigonocarpus pusillus (Brongn.) is 
a Tripterosperrmim . 
The other distinguishing feature of Tripterospermum , i. e. the more 
wing-like form of the ridges of the hard testa, remains to be considered. 
The difference would be best seen by a comparison of transverse sections of 
the body of the seed in the two genera. Comparing our common British 
form, described in this paper as Trigonocarpus Parkinsoni (see PI. XI, 
Fig. 5), with the figures given by Brongniart, it seems to agree (as was 
evidently thought by this author himself) rather more closely with Triptero- 
spermum than with Trigonocarpus. 
Should we, then, describe our British forms as belonging to the genus 
Tripterospermum ? We have already seen that the degree of prominence 
of the three principal ridges of the testa is the only remaining character by 
which to distinguish the two genera. Among our British forms there is 
some variation in this respect, not only in the principal ridges, but also 
in the less prominent ribs which occur between them, as will be more fully 
discussed later. Prof. Oliver, in his memoir on Stephanospermum , one of 
the radiospermic Gymnospermous seeds from St. Etienne, and in a com- 
parison of this form with other related Palaeozoic seeds, after pointing out 
that the seed described by Williamson as Trigonocarpon olivaeforme 
unquestionably belongs to the same affinity as the Hooker and Binney 
specimens, says : ‘ Consequently, for the present discussion, both these 
Trigonocarpons may conveniently rest in the Brongniartian genus Triptero - 
spermum ’ h Somewhat later, in his Notes on Trigonocarpus and Poly - 
lophospermum , discussing the same subject, Prof. Oliver says that ‘ In any 
case the structural feature used by Brongniart to separate Trigonocarpus 
and Tripterospermum seems unimportant ’ 2 . 
We concur in this opinion, and shall therefore describe our forms as 
belonging to Trigonocarpus. We are the more disposed to do this as this 
name has been extensively used in this country by Williamson and others 
for petrified specimens and casts similar to those described in this paper, 
and so has become well known to every one interested in fossil plants. 
Tripterospermum has never been used for British forms. 
We may now consider the question of the specific name which should 
be given to the common form. This question is made the more difficult of 
solution owing to the difficulty of accurately comparing the structural 
specimens preserved completely embedded in a matrix with the much more 
common form of preservation as a structureless cast. The specific names 
have usually been first applied to the casts and afterwards to structural 
specimens which appeared to agree in shape and size. 
1 Oliver (’04) (1), p. 391. 2 Oliver (’04) (2), p. 97 note. 
