220 THE POTOMAC OE YOUNGER MESOZOIC FLORA. 



Multitudes of these fragments occur in the layer mentioned before, and 

 they formed a considerable portion originally of the stratum They occur 

 here with Dloonltes BucManus, Brachyphijllum crassicaule, Williamsonia Vir- 

 ffiniensis. Baieropsis pluripartita, etc. Here again we find a remarkable case 

 of isolation. 



BRACHYPHYLLUM (Brongniart, ex parte), Schimper. 



Trees or shrubs, with limbs and twigs branching in an irregularly 

 pinnate manner; leaves very short, spirally arranged, densely crowded, 

 and touching by their broadened bases; when living probably fleshy and 

 pyramidal, curved upwards, or by mutual pressure acquiring a pentagonal 

 or hexagonal form, and projecting in the form of a very short nipple- 

 shaped boss, persisting for a long time; when old and on thickened 

 branches dilated into a shield-shaped form ; scars left by the fall of the 

 leaves erect, rhombic in form, contiguous, and marked in the middle by a 

 vascular cicatrix. 



The above is the description of the genus given by Schimper, exclud- 

 ing the form of PachyphyUum Saporta, Pagioj^hyllum Heer, formerly in- 

 cluded in the genus by Brongniart. He goes on to say that the leaves are 

 very short, fleshy, and shaped like a nipple, which may be either straight 

 or curved upwards, being enlarged and touching the adjacent ones at base. 

 With age, and in consequence of its dilation, the nipple-shaped projection 

 disappears almost wholly, and the leaves resemble pentagonal or hexagonal 

 cushions, which surround the branch like a cuirass. After the fall of 

 these singular leaves the branches are covered with rhomboidal scars like 

 those of Lepidodendron. 



This description in its essential points agrees better with certain of 

 the Potomac plants than that of any other known fossils. The Potomac 

 fossils show mostly only the scars left after the fall of the leaves, and tliey 

 do not have the vascular cicatrix in their center, but in all other respects 

 these scars are exactly like those of Brachyphyllum. The peculiar mode of 

 branching of this genus is well shown in the fossils now in question. In 

 the shape of the leaf-scars and in the arrangement of the branches, as well 

 as in their form, some of the Potomac fossils are much like Schimper's 



