2i2 STUDIES IN FOSSIL BOTANY 



same stem — in fact, the characteristic markings of the 

 type-species of Clathraria and Leiodermaria respectively, 

 5. Brardi and 5. spinulosa, have repeatedly been 

 found associated in this way. The two forms of surface 

 frequently occur in alternating zones on the stem, and 

 the same is the case with the Rhytidolepis and Favularia 

 markings among the Eu-Sigillariae. It has been sug- 

 gested that the crowded leaf-Scars characteristic of Favu- 

 laria and Clathraria may have coincided with periods of 

 slow growth, while the more scattered arrangement in 

 the Rhytidolepis and Leiodermaria forms may have been 

 due to seasons of rapid elongation. All these four 

 types, however, have been regarded, even by some 

 modern authors, as distinct genera. As a matter of 

 fact, only the division between Eu-Sigillaria and Sub- 

 Sigillaria holds good taxonomically, and even here the 

 distinction is not always a sharp one, for some forms of 

 Favularian and Clathrarian 1 surface approach each 

 other very nearly, 2 and may even occur on the same 

 stem. 



A hundred or more so-called species of Sigillaria 

 have been described, according to the superficial mark- 

 ings, but they are admittedly of doubtful value at the 

 best. The genus, however, was evidently a rich and 

 varied one, though the characters available are altogether 

 insufficient for the clear limitation of the different forms. 



Just as in Lepidodendron, so also in Sigillaria, the 

 appearance of the stem was often much changed by the 

 partial destruction of the external tissues, and the various 



1 Clathrarian Sigillariae are also spoken of as Cancellatae. 



2 On the subject of the surface-characters' of Sigillariae, see Zeiller, 

 " Revue des travaux de Paleontologie," Rev. ge'n. de botanique, 1897, 

 p. 404; EUments de Paliobotanique, Paris, 1900, p. 190. 



