STIGMARTA. 



267 



tion caused by pressure, to which the bases of the pits containing the 

 appendages were exposed. All authors, as far as I know, have either 

 silently or expressly accepted this explanation, though the circumstance 

 that such deep holes should have so entirely disappeared, always presented 

 some difficulty, especially in cases where from the perfectly regular 

 cylindrical form of the whole body the pressure, to which the surface must 

 have been exposed, is not quite obvious. That this view is erroneous has 

 been quite recently shown for the first time by Williamson 1 , who has 

 proved his point unanswerably. He has brought together a long series of 

 instructive specimens, some of which have been figured 2 . From these we 

 learn that we are dealing here not with the surface of the cast, but with 

 fragments of a peculiar mould. It will be shown more fully later on that 

 there was a broad tubular cavity present in 

 every appendage. Supposing then that these 

 appendages were destroyed and torn away 

 down to the preserved basal portion in a 

 specimen which was going to be covered up, 

 the latter will have been beset with these 

 basal portions, as with so many open cups ; 

 and at the bottom of each of these there 

 must have been a short thread-like process, 

 the remains of the single vascular bundle 

 which has been torn away. Then when the 

 object has been buried and the substance of 

 these cups has been changed into coal or has 

 entirely disappeared, if cast and mould are 

 separated from one another, every depression 

 will appear on the latter as a flat protuberance, 

 and the wall of every cup as an annular fissure 

 entering deeply into the mould. But the cone in the pit answers to 

 the cast which must be formed in the cavity of the cup, and necessarily 

 comes to an end at the point of insertion of the appendage, which 

 here corresponds to the margin of the orifice of the annular fissure. 

 The remains of the vascular bundle which appears at this spot must leave 

 behind it the central dot-like impression. As has been said, no doubt can 

 ever arise with regard to this explanation. The mould figured by 

 Williamson 3 should be compared with the diagrammatic representation 

 given here in Fig. 31. Williamson has also obtained analogous results 

 artificially from casts of suitable Stigmariae. On the occasion of my last 

 visit to Manchester I had opportunity through his kindness of examining 



FIG. 31. Stigmaria ficoides in the state 

 of preservation described by Hooker (1). 



A, fragment of Hooker's original figure. 



B, longitudinal section diagrammatically 

 represented ; the cast-side is black, the 

 opposite impression is white ; the latter 

 answers exactly to Hooker's description. 



Williamson (6). 



a Williamson (6), tt. 12 and 14. 



3 Williamson (6), t. 14. 



