PITYOXYLON 347 



Then they become slightly smaller and thicker-walled and pass gradually 

 into the fall wood. The resin ducts are very large. They are not found 

 in the summer wood, but occur irregularly in the early fall and late fall 

 wood. The medullary rays as observed in this section arc straight and 

 separated by 3-8 or to rows of wood cells. The individual cells are 

 apparently long. 



''Radial. Notwithstanding the fact that the wood seems to be perfectly pre- 

 served, it does not reveal the structure well in this section. The wood 



* cells are seen to be sharp-pointed where they join. They are, of course, 

 broad in the spring and summer wood and very narrow in the fall wood. 

 It is very difficult to make out the pits, but in exceptionally well-pre- 

 served portions a few may be faintly seen. They are scattered, but in 

 a single series. They are so obscure that no satisfactory measurements 

 can be made. The medullary rays in this section are long, thick-walled, 

 and without markings, so far as can be made out. 



" Tangential. This section is very plain. The medullary rays are numerous 

 and in a single series, although occasionally a ray may be observed in 

 which there are 2 series of cells for a short distance. In such cases 

 the cells are always smaller than the ordinary ray cells. The number 

 of cells making up each ray ranges from 2 to 30 or more, but the average 

 number is about 8-15. The rays in which there is a resin duct are 

 rather rare. The duct is large, taking up all the width of the ray. The 

 remainder of the ray is 3 rows of cells high in the middle, and is reduced 

 to i at the extremities. The wood cells show plainly in this section. 

 They are not provided with pits or other markings" (Knowlton). 



Trunks of large size, 3-5 feet in diameter. Material silicified. 

 Tertiary of the Yellowstone National Park, at Specimen Ridge, near head 

 of Crystal Creek, and Yancy Fossil Forest (Knowlton). 



44. P. amethystinum, Knowlton 



" Transverse. Much like P. Aldersoni except that the rings are narrower, 

 the cells of the spring and summer wood are smaller, and the late fall 

 cells have thinner walls. The resin ducts are also much the same, being 

 in general only a little smaller. A few are found in the summer wood, 

 but most of them are in the fall wood. The rays are not nearly so 

 numerous as in the last species. They are often separated by as many 

 as 25 rows of wood cells. 



"Radial. The radial section of nearly all woods from the Yellowstone 

 National Park is more or less obscure. The one under consideration 

 is no exception to this rule, and it is only after considerable search that 

 the pits can be determined. They are in a single row and are rather 

 small. They are so obscure that it is impossible to make trustworthy 

 measurements. The medullary rays, as seen in this section, are com- 

 posed of long, thin-walled cells, and, so far as can be determined, they 

 are without pits or other markings. 



" Tangential. This section shows the structure very plainly. The medullary 

 rays are abundant and always in a single series, except the large, com- 

 pound ones. The number of cells in each ray varies from 2 to 10 or 12, 

 the average number being about 6. The compound rays inclosing the 



