2 66 STIGMARIA. 



cast which encloses the other has taken the place of the peripheral portions 

 of the axis of the Stigmaria. This mode of explanation, which was sug- 

 gested long ago by Steinhauer *, and has fyeen accepted by all authors, is 

 in agreement \vith the fact that the interior cylinder is never found in a 

 central position, but lies excentrically almost touching the surface of the 

 surrounding cast on one side, and that when the entire axis is sunk in and 

 pressed flat, the cylinder is always in the middle of the one broad side of it, 

 having reached that position by its own weight. The Stigmaria-casts are 

 very often firmly connected by a narrow strip of substance on one side to 

 the rock in which they are imbedded, and then they do not fall out of their 

 cylindrical mould. In such cases the cylinder of cortical tissue must have 

 been split longitudinally when the mould was filled, and through this fissure 

 the cast was brought into direct communication with the surrounding matrix ; 

 displacements also were usually produced and longitudinal foldings of the 

 surface, and these serve as characteristic marks. From specimens of this 

 kind, either through the operation of running water or if they were split by 

 pressure, the interior cylinder might obviously slip out on one side, and it is 

 sometimes found free in the rock. This happens occasionally in the Culm of 

 Burbach. I have a specimen of the kind myself twelve centimetres in length, 

 in which the characteristic features of the surface are most distinctly to be 

 seen. Good figures of such casts are given in Williamson 2 and in Schimper 3 . 

 Further indications of original structure are occasionally observed in the 

 substance of the casts, but we must defer any notice of these till we come to 

 speak of the anatomical structure. 



Hooker 4 described in 1848 two small fragments of Stigmaria from the 

 English Coal-measures, which are distinguished by the particularly sharp 

 preservation of the surface and depart in essential points from the 

 ordinary character. These specimens are not round casts, but irregular 

 fragments with scars on one side only. The scars, which ordinarily lie in 

 slight depressions, here occupy the apex of flat protuberances. In each of 

 these protuberances is sunk a smooth-walled crater-like cavity rather 

 broader below, which penetrates to a depth of six millimetres and resembles 

 the canal of a boring animal. The exterior margin of the orifice is slightly 

 raised and thickened. From the bottom of each pit rises a conical body 

 nearly filling the pit, which narrows a little towards the upper part, and 

 being broken off transversely exactly at the orifice, shows on the surface of 

 fracture a slightly depressed central trace-point. According to Hooker 

 these cones are the basal portions of the appendages, the projecting portion 

 having been broken off above the depression. The different appearance of 

 the ordinary specimens before described is referred by him to the deforma- 



1 Steinhauer (1). 3 Williamson (6), tt. 13, 14. * Schimper (1), t. 119, f. n. 



1 Hooker (4). 



