Uiyper Palaeozoic and Mesozoic Fossils. 311 



strong uietliau ridge corresponding to the grooving of the opposite 

 surface (PI. XXVII., Fig. 2). 



Another specimen noticed in the shale seems to be a small 

 sporophyll, such as might be found closely grouped around the 

 axis of the strobilus (PI. I., Fig. 3). It shows the characteristic 

 liollow for the reception of the sporangium. 



Remarks on variation in the form of the leaf-aishio7is in 

 Lepidodendron australe. — There are two specimens of li. australe 

 in the present collection, occurring in a grey-coloured and 

 limonitic shale, both of which show the transversely elongate form 

 of the leaf-cushions^ ; a modification undoubtedly due to the fact 

 that they represent older portions of the stem. On this point 

 Kidston may be quoted'^ : — "In most species the increase in the 

 girth of the stem has a tendency to produce a greater lateral 

 increase in the proportion of the leaf-scars than is equalised by 

 the upward growth of the trunk ; consequently in some species, 

 where the leaf-scars on the young twigs have a vertical length 

 much greater than their width, their older conditions show 

 proportionally a much greater transverse diameter.^' 



The average relative diagonals of the rhombic leaf-cushions in 

 the present specimens are : — 'LVansverse, 13.5 mm.; vertical, 10.5 

 mm. An extreme form in another exposed fragment on one of 

 these blocks measures 16 x 9 mm. That the variation of the 

 relative elongation of the rhombic areas is due to the age of the 

 stem is proved in the case of this Australian species by measure- 

 ments which I have taken on the relative diagonals of the leaf- 

 cushions of the type-specimen of L. australe, in the National 

 Museum, a portion only of which was figured in the Prodromus 

 by McCoy. Near the base of this type-specimen the leaf-cushions 

 measure 10.5 mm. trans., by 8 mm. vert. Close to the top of the 

 left branch they become almost equilateral, measuring 8 mm. 

 trans., by 7.5 vert. The two points at which measurements were 

 taken are 21 centimetres distant from one another. 



The conspicuous depression usually seen near the upper angle of 

 the leaf-cushion in L. australe must be regarded as the opening 



1 " Leaf-sears " of some authors ; a term applied more correctly to the upper portion of 

 the area covered by the leaf base. 



2 Cat. Palaeozoic Plants in the British Museum, Loud., 1886, p. 152. 



