84 Arber. — A Revision of the Seed Impressions of the 
say is that some species of Samaropsis and Cordaicarpus belong to the 
Cordaitales, and that, of the other genera, some or all species may belong to 
the Pteridosperms. The platyspermic seeds appear to be the more varied 
type, nearly double the number of genera and species of these seeds, as com- 
pared with the Radiosperms, being recognized here. 
I will begin with Trigonocarpus , the best known Coal Measure seed, 
both as an impression and as a petrifaction. This is a happy case in which 
it has been possible to correlate both types of preservation very exactly. 
Though Trigonocarpus P arkinsoni , Brongn., stands secure, the other types 
which have been referred to this genus have caused trouble and confusion, 
and may continue to do so for some time yet. 
Trigonocarpus , Brongn. 
T. P arkinsoni , Brongn. This species has been so thoroughly investi- 
gated by Scott and Maslen 1 that a bare mention of it here is all that 
is necessary. As an impression this seed occurs in three states of preserva- 
tion. 
Forma a. This shows the external surface of the sarcotesta, the true 
outer surface of the seed. It is the ancient Carpolithes alata of Lindley and 
Hutton, and is one of the most unmistakable of Coal Measure fossils. 
Forma ft (PI. VI, Fig. 2) shows only the sclerotesta and micropyle. 
In impressions the latter is frequently covered up by the rocky matrix, and 
in this condition it is sometimes difficult to recognize this state. The 
surface of the sclerotesta, in my experience, is smooth, as in Fig. 2, and 
a difficulty also arises here, as Scott and Maslen have pointed out, 2 for 
in the petrified specimens the outer surface of the sclerotesta possesses 
twelve longitudinal ridges 3 of unequal prominence. I am unable to throw 
any light on this matter. This state was originally described as Rhabdo- 
carpus Bochschianus by Berger. 4 
Forma y (PI. VI, Figs. 3-5), the well-known, tri-ridged casts of the 
nucellus or seed cavity. This is the state to which both the specific and 
generic names were first applied in 1828. So far as I am aware these 
casts never show any sign of splitting at the pointed end, or of a definite 
foramen at the rounded basal end. The latter is sometimes quite smooth 
(Fig. 4, lower seed), or sometimes there is a small, projecting, oval umbilicus, 
as in Figs. 4 (upper seed) and 5, which Scott and Maslen 5 regard as probably 
indicating ‘ the edge of the tracheal disc from which the free nucellus sprang \ 
I have, however, never seen any definite foramen disclosing an inner cast, 
and this, as we shall see, is important in relation to some other casts, which 
have so far been referred to the same genus. 
1 Scott and Maslen (’07), p. 96. 2 Ibid., p. 130. 3 Ibid., p. 106. 
4 Berger (’48), p. 21, PI. I, Figs. 13, 14. 5 Scott and Maslen (’07), p. 128. 
