240 
$ 9. Trigonocarpus Shorensis Salisb. 
Literature: Salisbury (126). 
Figure: 126 p. 42 :and:43; 
As regards the organisation of this seed, it is nearly 
the same as Trigonocarpus Parkinsoni. 
8 10. Mitrospermum compressum Will. 
Literature: Arber (2). 
Figure: 2 p, 494 and 495. 
The palaeozoic seeds may be divided into two groups 
the Radiospermae, which are in transverse section radial- 
symmetrical, and the Platyspermae which are bilateral- 
symmetrical. To the first group the seeds belong described 
in the foregoing pages: Lagenostoma, Sphaerostoma, 
Physostoma and Trigonocarpus, to the latter belongs 
Mitrospermum. 
Mitrospermum (Cardiocarpon) compressum has a length 
of about 5 mM and is flattened in the plane of symmetry. 
The testa is composed of three layers: sarcotesta, 
sclerotesta and endotesta. The first forms a thin layer of 
delicate tissue on the sclerotesta, but in the principal plane 
(the plane in which the seed has its greatest diameter) it 
becomes flat in order to form a broad wing. The most 
conspicuous feature of the sarcotesta is an outer layer of 
: large mucilage cells, the hypoderma, which is covered, in 
the best preserved specimens, by a thin small-celled 
epidermis. 
In the principal plane the sclerotesta is divided into 
two valves, and it is possible that these valves ultimately 
separate, though there is not such a clearly defined 
dehiscence plane, as in the seed Diplotesta (135). On both 
valves in the secondary plane two less prominent ribs are 
