452 STUDIES IN FOSSIL BOTANY 



sporophyll had scarcely even begun in a case like 

 this. 



Mr. Kidston was thus the first to observe direct 

 continuity between the seed and the frond in a Fern- 

 like Palaeozoic plant, and it was his discovery, in 

 conjunction with the equally strong though less direct 

 evidence in the case of Lyginodendron, which first 

 suggested the institution of the class Pteridospermeae. 



There is at present no instance of a petrified seed, 

 showing structure, which can be referred to the Medul- 

 loseae or Neuropterideae with the same certainty as 

 Mr. Kidston's casts. One of the best-known Palaeozoic 

 seeds, however, Trigonocarpus Parkinsoni, Brongn., may 

 be attributed with great probability, as we shall see, 

 to Alethopteris ; we will therefore shortly describe 

 its structure. 



Trigonocarpus Parkinsoni is found in three distinct 

 states of preservation in the English Coal-measures : 

 (i) in the common condition of nut-like, somewhat 

 triangular casts, which, as shown by Hooker and 

 Binney and by Williamson, are really internal casts 

 of the seed cavity ; (2) as external casts, showing the 

 testa and the true form of the seed ; (3) as petrifactions, 

 in which the structure is more or less perfectly preserved. 

 From these various data a fairly complete knowledge 

 of the organisation of the seed has been gained ; its 

 more important features are shown in the diagrammatic 

 figures 170-172. 



The seed is a very large one, the length reaching 

 5 cm., of which quite half is accounted for by the 

 enormously long micropyle ; the body of the seed has a 

 maximum diameter of over 2 cm. (Fig. 1 70). The testa 



