138 LYCOPODIALES [CH. 



is merely a fertile condition of Lepidophloios and possibly of 

 other lepidodendroid plants. He was also inclined to regard 

 the Halonial tubercles as younger stages of the larger scars 

 characteristic of the genus Ulodendron. Williamson's contribu- 

 tions to our knowledge of Halonia are of primary importance ; 

 he supplied further proof of the Lepidodendroid nature of these 

 branches and advanced our knowledge of their anatomy. In 

 an early paper* he expressed the view that the differences on 

 which Halonia and Ulodendron are separated are such as result 

 from a difference in age and are not of generic importance. 

 In the last memoir, of which he was sole author, published by 

 the Koyal Society^ Williamson brought forward further evi- 

 dence in support of this well-founded opinion. 



That the fossils known as Halonia are branches of a 

 lepidodendroid plant is at least certain, and it is probable that 

 the lateral branches which they bore were fertile, though satis- 

 factory proof of this is lacking. We know also that Halonia 

 branches are characterised by the Lepidophloios form of leaf- 

 cushion ; there is, however, no sufficient reason to assume that 

 such branches were never attached to stems with the cushions 

 of the Lepidodendron form. The further question, namely 

 whether Williamson was correct in his contention as to the 

 absence of any essential distinction between Ulodendron and 

 Halonia, does not admit of an unchallenged answer. In 1903 

 Weiss' described the anatomy of a specimen of a biseriate 

 Halonia branch of Lepidophloios. The form of the leaf-cushions 

 is unfortunately not very well preserved, but Weiss figures 

 other specimens with two rows of tubercles on which the leaf- 

 cushions are sufficiently distinct to justify a comparison with 

 those of Lepidophloios. He believes with Williamson that it is 

 the presence of tubercles in place of scars which distinguishes 

 Halonia from Ulodendron, and that the arrangement of the 

 tubercles or scars is a matter of little importance. He ex- 

 presses the opinion justified by the evidence available that the 

 absence or presence of tubercles is merely due to accidents of 

 preservation or, one may add, to difference in age. Kidston* 

 dissents from Weiss's description of his specimen as a biseriate 

 1 Williamson (72). = ibid. (93). 3 Weiss, F. E. (03). * Kidston (05). 



