DAWSON — SIGILLARIA, CALAMITES, AND CALAMODENDRON. 149 



Tho exception above referred to is, that in one tree, which from 

 its nijirkings 1 sujjposed to be a SujillHrIa, tho woody tissue was com- 

 posed of larjifo cells, with many rows of pores (" multiporous tissue " 

 of ray former pajjors) of tho ty])e of that to which Trof. Williamson 

 has given tho name of JJictifo.ri/lon*. Since, however, as Prof. Wil- 

 Kamson has well shown, such tissue may be regarded as a modifica- 

 tion of tho discigerous variety, and since Corda long ago found it in 

 the axis of a species of iStii/ttutriaf, there is nothing improbable in tho 

 supposition that we have hero merely an indication of a spccitic or 

 ■ubgeneric difference coming within the limits of the genus Si(jilU{ria, 

 as at pr<;sent understood. 



It is to be observed that most of the erect trunks in tho cool-for- 

 mation have not preserved their external markings with sufficient 

 distinctness to allow the species to be determined by the leaf-scars ; 

 but they show in most cases the characteristic ribs and rows of 

 punctures or areoles, modified in the manner wliicli is usual in tlio 

 case of old trunks of these plants near their bases J. 



In riate VIII. figs. 12, 14, 15, 10, 1 have represented some of tho 

 more usual forms of tissue in the erect ^igiflari<t'. 



In riate X. I have represented the best-i)reserved axis in my 

 possession. Fig. 23 shows the structures in the entire stem, except 

 the portions of cellidar bark lost by decay. In the centre is a IStern- 

 bert/la-inih(a). This is surrounded by a woody cylinder (i), tho 

 inner part of which (fig. 24, hi) consists of scahiriform tissue pass- 

 ing towards the outer surface into pseudo-scalariform {b 2), reticu- 

 lated with pores (b 3), and discigerous (b 4). This woody axis has 

 medullary rays (figs. 25, 20, 27), and is traversed by bundles of 

 scahiriform tissue proceeding from tho imier part of the cylinder. 

 The outer portion of the inner bark, not seen in this specimen, but in 

 similar prostrate stems, is composed of elongated thin-walled bast- 

 cells, with somewhat obtuse ends, and of larger diameter than the 

 woody fibres of the axis (fig. 29). The tissues of tho woody axis 

 are all arranged in radial series (fig. 28). 



Prostrate Trunks. — In the coarse shaly coals, and in the roof- 

 shales of the ordinary seams, there are often flattened stems of 

 SigiUaria, having the tissues partially infiltrated with carbonate of 

 lime or carbonate and sulphate of iron. The tissues usually pre- 

 served in these flattened trunks are those of the bark, and more 

 especially its large bundles of elongated or prosenchymatous cells 

 (" bast-tissue "). Of this I have been enabled to obtain very perfect 

 specimens from these flattened tninks. In a few instances only the 

 woody structure of the axis remains, showing the same descriptions 

 of wood-cells already referred to as characteristic of the erect trees. 

 Plate VIII. fig. 11 is an example of the structures in one of these 

 prostrate stems. 



* Trivns. Royal Micro. Soc, Aug. 1809. 



t Beitriige zur Flora &c. 1845, pi. xiii. Corda regards this na the structure 

 of 5. Jiroides, and the more ordinary variety as tliat of S. ani. hra. 



\ Tlie species wliicli I have described as <S'. Brownii, Acad. ueol. 2nd edition, 

 may be regarded as a representative of these trees. 



