o/" Ulodendron to Lepidodendron, (be. 175 



known. In Lepidodendron and Sigillaria * it consisted of 

 cones, and in Rhytidodendron it was probably of a sirailai' 

 nature. In certain Sigillarioe and Rhytidodendra {R. punc- 

 tatum) there is also occasionally a small tubercle above the 

 leaf-scar or on some other part of the back {8ig. spinulosa^ 

 Germar)t. 



The evidence afforded by the various specimens which have 

 been described in the earlier part of this communication, as 

 well as that derivable from examples figured and described 

 by previous writers on this subject, may now be summarized, 

 when I hope to show that plants not only belonging to Lepi" 

 dodendronj but also to Sigillaria and Rhytidodendron^ possess 

 large scars, arranged on their stems in two opposite vertical 

 rows. 



Lepidodendron Veltheimianum, Sternberg. 



It has been stated by some authors (Brongniart, Schimper, 

 and others) that the leaf-scars on the stems of Ulodendron 

 were always smaller than those occurring on the stems of 

 Lepidodendron of a similar size, and also that the vertical 

 clefts which occur in the bark of Ulodendron were not com- 

 mon to Lepidodendron. This may be a general rule, but it 

 is by no means a universal one. The size of the leaf-scars on 

 those so-called Ulodendra is very much infiaenced by the 

 extent to which the bark has become longitudinally fissured, 

 for these clefts act as escapements to the lateral strain (if not 

 produced by it), which is caused by the increase in girth of 

 the stem ; whilst in those cases where the bark does not become 

 fissured, the leaf-scars are laterally expanded by the same 

 strain which produces the Assuring of the bark. 



On specimen No. 1, PI. III. fig. 1, it will be seen that the 

 leaf-scars have attained a considerable size, and they cannot 

 be said to be characteristically smaller than those of ordinary 

 Lepidodendra, nor do they in any point differ in shape or 

 arrangement from characteristic leaf-scars of Lepidodendron 

 Veltheimianum. It is also to be noted that in this example 

 the bark is more than usually free from the vertical fissures 



* See Zeiller, " Sur des Cones de Fructification des Sigillaires" (' Comptea 

 Rendus des Seances de I'Academie des Sciences,' 30 June, 1884) ; also 

 " Cones de Fructification des Sigillaires " (Ann. des Scienc, Nat. 6'' ser., 

 Bot. tome xix. p. 256, pis. xi., xii.). 



t I ha-ve not entered into the differences in internal structure between 

 Lepidodendron, Sigillaria, and Ulodendron. Suffice it to say, that the 

 internal organization of Lepidodendron and Sigillaria is distinctly dif- 

 ferent, and that the few specimens of Ulodendron Yrlcath have been found 

 with their internal structure preserved apparently agree with Lepido' 

 dendron, 



13* 



