420 OLDER MESOZOIC FLORAS OF UNITED STATES. 
The following is his description of the genus and the species: 
DESCRIPTION OF A NEW GENUS AND SPECIES OF FOSSIL WOOD FROM THE JURASSIC OF THE BLACK HILLS. 
By F.H. KNowLTon. 
Genus PINOXYLON Knowlton nov. gen. 
Internal structure of the wood same as in Pinus, except in the 
absence of fusiform rays. 
PINnoxYLON DacoTENsE Knowlton n. sp. 
Pl. CLXXIX. 
Trunks of medium size; annual rings broad, very distinct; tracheids 
of spring and summer wood very large, thin walled, more or less 
hexagonal in shape; tracheids of fall wood thick walled, elliptical in 
outline; bordered pits on radial walls of tracheids, mainly in spring 
wood, of large size, mostly in two rows, rarely in a single row; 
medullary rays in a single series; resin cells wanting; resin passages 
present, scattered, mainly in the fall wood. 
Transverse section: In this section the annual rings show very 
plainly, even to the naked eye, being from 2 to 4.5 mm. in width. 
The distinction between the spring and fall wood can also be seen with 
the naked eye, the former appearing as broad white bands and the 
latter as dense black bands of varying width. Under the microscope 
the line of demarcation between its fall and spring wood is observed 
to be very sharp, indeed. The fall wood consists of thick-walled cells . 
of an elliptical or oblong outline and rather loosely placed, as may be 
seen from the figure (Pl. CLX XIX, Fig. 1). The succeeding spring 
wood is composed of very large cells with relatively thin walls. 
The medullary rays as shown in this section (Fig. 1) are long and 
quite thick walled. As far as could be ascertained from the sections 
made there are no resin cells in this wood. The resin passages, how- 
ever, are present and quite numerous. They do not seem to be con- 
fined to any particular portion of the ring, but are scattered, being, 
perhaps, most abundant in the fall wood. They are of relatively 
large size and lined with thin-walled epithelium cells (Fig. 2). 
Radial section: There is much to be seen in this section. The walls 
of the cells of spring and summer wood are preserved in most cases 
with two rather irregular rows of large bordered. pits. In rare cases 
these pits are in a single row, as shown in Fig. 3. The average size 
of the outer circle is .025+, that of the inner circle about .015-+. The 
rays are seen to advantage in this section. The cells are rather long, 
covering the width of usually some four or more cells of the spring 
wood. They are rather thick walled, the walls being strongly dentate 
or somewhat irregularly thickened. This irregular thickening is well 
shown in the figures. The ray cells are provided with a few scattered 
