18G THE POTOMAO OR YOUNGEK MESOZOIO FLOEA. 



The specimens of this variety are much rarer than those of the normal 

 species B. Buchianm. 



Tliis form, if identical with the European fossils named above, begins 

 in the Wealden and survives into the Neocomian. 



TYSONIA, o-en. nov. 



IVunks varying considerably in shape and in dimensions, petrified with 

 silica, uii.re or less flattened, seen with the broader side in front, tliey are 

 oblong-ovate and truncate ; in cross-section they are broadly subelliptical ; 

 medulla proportionally small; woody cylinder C(jraparatively thick; cor- 

 tical exterior layer, with the permanent bases of the petioles very thick ; 

 bases of tlie petioles in cross-section normally subrhombic or subtriangulai', 

 with the lower angle very obtuse, the outer angles acute and prolonged, the 

 superior side forming a curved line bent upwards or forming an obtuse 

 angle, but often from pressure distorted into irregular, rliombic, or trian- 

 gular forms; trunks each with a large eccentric terminal leaf-bud or growing- 

 bud ; some of the trunks, probably of female plants, have numerous lateral 

 buds ; others, probably male plants, are without lateral buds ; bases of the 

 petioles of the leaves represented by open casts of the petioles, which from 

 the points of insertion of the petioles pass upwards and outwards, so that 

 their direction at their exit from the cortical investment is nearly horizontal. 



These trunks have some of the characters of Carruthers's genus Bennett- 

 ites and of Maiitellia Brongn., being apparently an intermediate type. I 

 have named the genus in honor of Mr. Tyson, who first called attention 

 to these plants. Carruthers' says: "Through the kindness of Principal 

 Dawsi^n I have seen a photograph of one of the cycadeaii stems found by 

 P. Tyson, esq., in Maryland. * * * It is obviously a species of 

 BenndtUes, with smaller leaf-scars than those in B. Saxhijcaius" 



It seems from his description of the plant that Carruthers saw a 

 photograph of trunk No. 1, He does not seem to have had a photograph 

 of trunk No. 2, which difters in important points from No. 1. Carruthers 

 thinks that the apparent buds on his Beimeltltes are not leaf-buds, but 

 organs of reproduction. There is nothing in the Potomac plants to show 



' Fossil cycadean stems from the secondary rocks of Britaiu, Trans. Linn. Soc. London, vol. 2(>, 1 "~l, 



postscript, p. 708. 



