DESCKII'TIOX OK THE SPECIES. 187 



wliat the natiuo of the buds was. Oarrutlicrs's description of tlie cross- 

 section of the bases of the petiole of Benrnttites agrees well with the cross- 

 sections in Ti/sonia. lie explains the e.\tenor aspect of the trunks, which 

 shows a series of hollow imprints left by the bases of the petioles separated 

 by plates, as caused by certain processes which seem to have taken place in 

 the Potomac plant also. He says the trunk, when living, was surrounded 

 by the permanent bases of the petioles, and the base of each petiole was 

 clothed with a dense ramentum, which was developed to such an extent as 

 to separate very considerabl}' each petiole from its neighbor. The silica 

 forming the petrifying material having found speedy access to the delicate 

 scales of the ramentum has preserved them in a remarkably perfect manner- 

 In some of the English specimens the petrified ramentum stands out as a 

 net-wurk of ridges, while the originally more durable petioles have decayed 

 and left rhomboidal cavities representing their original form. Carruthers 

 states also that the leaf disarticulated at a point some distance from the 

 cortex, leaving a considerable portion of the bases of the petioles perma- 

 nently surrounding the stem. This explanation given for the English 

 Bennetites no doubt applies to the similar features seen in the Potomac 

 Tysonia. In this latter, now thoroughly silicified in all the specimens seen, 

 it is clear that the material surrounding the bases of the petioles was silicified 

 before the bases, and that these subsequently decayed, leaving perfi'ct 

 hollow casts of their form. 



No microscopic examination of the trunks of Tysonia has been made, 

 and I am compelled to confine the description to macroscopic characters. 



There are two kinds of trunks, diff"ering mainly in the presence or 

 absence of axillary buds There are two kinds of axillary buds on the 

 same trunk, a feature which, according to Carruthers, appears in MantcUia 

 but not in Bcnnettltes. In would seem that in the buds on Ty.'ionla, which 

 resemble those of BenneUites, there was a woody axis of varying size, 

 surrounded by scars of leaves much smaller than the normal ones, or of 

 bracts. This woody axis certainly protruded beyond the bases of the 

 petioles. Some of them are larger than others and some arc hollow, the 

 hollows being no doubt due to decay before silification. No indication of 

 fructification has been seen in connection with the Potomac stems. 



