XV] LEPIDODENDRON 103 



been dealt with in a paper by Barroisi who discusses in detail 

 the habitats of these small animals from the point of view of the 

 conditions under which the plants were preserved. In a note by 

 Malaquin appended tb Barrois' paper the belief is expressed 

 that Spirorbis lived on pieces of Palaeozoic plants which lay 

 under water. 



The fact that with one exception all the Spirorbis shells on 

 the specimen of Lepidadendron, of which two leaf-cushions are 

 shown in fig. 146, E, occur on the large parichnos scars on 

 the cheeks of the cushions, suggests the possibility that the 

 escape of gases from the parichnos tissue may have rendered 

 the position attractive to the Spirorbis. It can hardly be 

 accidental that the shells occur on the parichnos strands. This 

 fact recalls the view held by Binney" and accepted with favour 

 by Darwin' that Lepidodendron and other coal-forest trees may 

 have lived with the lower parts of the stems in sea water. 



Above the leaf-scar is a fairly deep triangular or cresentic 

 pit (fig. 146, C, I) known as the ligular pit from the occurrence 

 on younger shoots of a delicate organ like the ligule of Isoetes 

 (fig. 132) embedded in a depression in the upper part of the leaf- 

 cushion. The ligule was first figured in Lepidodendron by 

 Solms-Laubach^ and described in English material by Williamson 

 under the name of the adenoid organ ". 



In some Lepidodendron stems a second triangular depression 

 may occur above the ligular pit, the meaning of which is not 

 clear: this has been called the triangulum by Potoni^*. Stur' 

 suggested that it may represent the position occupied by a 

 sporangium in Lepidodendron cones. 



It is important to remember that as a branch increases in 

 girth the leaf-cushions are capable of only a certain amount of 

 growth : when the limit is reached they are stretched farther 

 apart and thus the narrow groove which separates them is con- 

 verted in older stems into a comparatively broad and flat 

 channel, thus altering the surface characters. 



1 Barrois (04). See also Etheridge (80) ; Geikie (03) p. 1049. 



2 Binney (48). ^ Darwin (03) vol. ii. pp. 217, 220. 

 <" Solms-Laubach (92) PI. ii. figs. 2, 4. » Williamson (93) p. 10. 



6 Potoni6 (05) Lief, iii., p. 41. ' Stur (75) A. Heft ii. p. 277. 



