208 ANATOMY OF THE GYMNOSPERMS 



in the genus Araucarioxylon. They are much smaller than the pores 

 on the radial walls, and are in a single or rarely in 2 series. The pores 

 are always separated from each other, sometimes widely so. The diame- 

 ter of the outer circle is about .0075 mm., and that of the inner about 

 .0027 mm. The medullary rays are numerous and range in height from 

 1-22 cells. It is possible that in some rare cases they may be in 2 

 series, but this is certainly not commonly the case. No resin ducts 

 have been detected in any of the sections, their absence being a well- 

 known character of the genus" (Knowlton). 



Material silicified. Specimens are represented by fragments of trunks 

 upwards of 20 inches in length and 1 3 inches in diameter. 



Triassic or Lower Jurassic near Fort Wingate, New Mexico ; Lithodendron 

 Creek (Cretaceous?) and Chalcedony Park, Arizona (Knowlton). 



8. * * A. Hoppertonae, Kn. 



" Transverse. Annual rings very distinct, of 4-8 rows of very thick-walled 

 fall wood. The spring wood is made up of very large though thick- 

 walled cells, which begin very abruptly at the fall wood. The cells 

 gradually decrease in size until the last 5-8 rows of cells are very thick. 



" Radial. As the material has been very finely preserved, this section shows 

 remarkably well. The wide cells of the spring wood are provided with 

 usually 2 longitudinal rows of hexagonal pores, which quite cover the 

 walls. Occasionally, in cells of unusual width, the pores, while in 2 

 series, are only slightly compressed. Usually when but i row is present 

 they are hexagonal and occupy the center of the cell. The inner pores 

 in these punctations are relatively small and slightly elongated in a 

 direction at right angles to the cells. The medullary rays as seen in 

 this section are short, covering the width usually of 4 or 5 wood cells, 

 although occasionally longer and covering as many as 8 cells. They 

 are provided with a single row of bordered pores so arranged that i 

 comes over each wood cell, or occasionally there may be 2 over a wood 

 cell. The inner pore is minute. 



" Tangential. The medullary rays as seen in this section are quite numer- 

 ous, in a single series of from 3 to sometimes as many as 15 superim- 

 posed cells. The wood cells as seen in this section do not seem to 

 have been provided with punctations or other markings" (Knowlton). 



Material silicified. 



Cretaceous formation of the Black Hills, Cycad bed 2 miles southeast of 

 Minnekahta Station, South Dakota (Knowlton). 



9. * * A. Edvardianum, Dn. 



" Trunks with distinct rings of growth, and with a central pith not 

 observed to have transverse laminae. Wood cells with i or rarely 2 rows 

 of contiguous, hexagonal areolas. Medullary rays simple, infrequent, with 

 2-10 rows of cells superimposed" (Dawson). 



Triassic of Prince Edward Island (Dawson). 



