184 THE POTOMAC OR YOUNGER MESOZOIC ELORA. 



All the parts are shown except the base of the leaves. Among the hun- 

 dreds of specimens seen, it is singular that in no case was any portion of 

 the base found. 



The leaves appear to have been thick and durable, for in a number of 

 cases the leuf-substance may be stripped from tlie stone, retaining the shape 

 of the leaflets. The epidermis seems to be especially thick over the mid- 

 rib. The nerves are immersed in the leaf-substance and are usually 

 obscured. 



In Ettingshausen's figure the specimen is somewhat distorted by press- 

 ure. The leaflets on the right-hand side have been crushed over at base 

 so as to appear to be attached to the upper face of the midrib and partly 

 within its margin, whereas they are really attached to the margin and lie 

 usually in one plane. 



This plant seems not to have been confined strictly to the Urgonian, 

 but to have survived to a later period, while its narrower form at least 

 began in the Wealden. Ettingshausen ' gives figures of Pterophyllum Sax- 

 onicum Reich., which certainly represent portions of I). Biichianus. In 

 these figures some of the leaflets go off at a more open angle than is usual 

 in D. BucJiianus, but this is due to distortion from pressure, for this mode of 

 departure can be duplicated in many of the Potomac plants. Ettingshau- 

 sen says that the surface of the Niederschoena ])lant is covered with fine 

 dots arranged in rows between the nerves. In the Potomac plants they 

 are closer than Ettingsliausen's figure represents them to be. Hosius and 

 Von der Marck- represent a fragment which they identify correctly with 

 Fterophijllum Saxoniauii, coming from the Neocomian sandstone of the Teu- 

 terburger Wald. This is evidently a portion of the midrib and the bases 

 of two leaflets of Diooidtes BucJiianus. The leaflets are by distortion from 

 pressure made to go off nearly at right angles. 



DlOONITES BUCHIANUS, var. OBTUSIFOLIUS. 

 Plate CLXVIII, Fig. 3. 



Leaves long and wide, with a stout midrib ; leaflets subopposite, 

 inserted on the upper face of the midrib and considerably within its mar- 



' Kreidellora vou Nieilerscliceua in vSachseu, PI. I, Figs. 11, 12. 

 ■ Flor. der Wcstfal. Kreideformatiou, PI. XLIV, Fig. VJa. 



