

A. C. Seward — Variation in Sigillarice. 215 



difficult to separate the species of this series from one another." 

 He goes on to describe Gerrnar's 1 S. spinulosa, characterized by an 

 even surface and peculiar small scars occurring below the leaf- 

 scars, probably representing points of attachment of roots ; these 

 scars, which gave rise to the term spinulosa, are found to be by no 

 means constant in their occurrence. Some specimens are mentioned 

 by Weiss, from the Halle Collection, which agree in all points with 

 S. spinulosa, except that the root (?) scars are absent ; the surfaces 

 of these specimens are marked with longitudinal and transverse 

 wrinklings. Immediately below the leaf-scars the transverse wrink- 

 lings appear more prominent than the longitudinal, and this region 

 becomes to some extent emphasized or distinct. In all the specimens 

 these transverse and longitudinal wrinklings occur more or less 

 pronounced ; the subquadrate form of the leaf-scar is another fairly 

 constant character. On examining the series of specimens Weiss 

 noticed that the leaf-scars were farther apart in the forms with an 

 even stem surface, and that the distances between them diminished 

 as the leaf-cushions became more and more developed, or. in other 

 words, as the Clathraria character became more marked. Four 

 figures are given, showing a passage from a Leiodermarian form in 

 which the wrinklings on the surface of the stem are very conspicuous, 

 and in which there is an indication of undulating lines between the 

 leaf-scars, through an intermediate form, called by Weiss S. Wet- 

 tensis, to the typical S. Brardi. As the leaf-cushions become more 

 and more completely formed, the surface wrinklings become less 

 and less conspicuous. Clear evidence is given of an intimate con- 

 nexion between the two sections Leiodermaria and Clathraria. 



In the Neues Jahrbuch fur Mineralogie, etc. (Jahrgang 1889, 

 p. 376), Weiss brings forward further evidence of the absence 

 of any definite line of demarcation between Leiodermarian and 

 Clathrarian types. A piece of thick, somewhat flattened stem, is 

 described from the collection in the Halle Museum, showing on one 

 side the Clathrarian type S. Brardi, and on the other side the 

 Leiodermarian type S. spimdosa. A second specimen is referred to 

 from Halle which shows a passage from a form resembling S. 

 Defrancei (Brongniart), one of the Clathrarice, to a Leiodermarian 

 form. The facts adduced by Weiss are considered by him to show 

 that in certain cases the Leiodermaria form of surface represents 

 a later, the Clathraria form an earlier state of the plant's growth. 

 This does not, says Weiss, hold good probably in all cases; possibly 

 many Sigillarice have Leiodermarian characters during the whole 

 of their life, and others retain Clathrarian characters. Some cases, 

 however, occur where the two types are simply expressions of 

 differences in age or development. 



Weiss divides the Sigillarice into two main groups : — 



A. Subsigillarice. B. Eusigillarice. 



1. Zeiodermarice. 2. Cancellatce. 3. Favularim. 4. Rhytidolepis. 



1 Die Versteinerungen des Steinkoblengebirges von Wettin und Lobejiin. taf. xxv. 



