242 ANATOMY OF THE GYMNOSPERMS 



" Tangential. The medullary rays have very small cells which have a 

 long diameter of .ois-.oiy mm. and a short diameter of only .01 mm. 

 The walls of the tracheids are so thick and the rays so small that 

 the walls between which they appear are but slightly bulged. The 

 tracheids do not exhibit pits on the tangential walls" (Knowlton). 



Remains silicified. The specimen is represented by numerous fragments 



of stem, upwards of 25 cm. in length. 

 The Potomac Formation of Dutch Gap and Nebasco Creek, Virginia 



(Knowlton). 



18. C. elongatum, Knowlton 



" Transverse. Annual rings apparent to the naked eye, but faint, i-6mm. 

 broad. The layer of the fall wood is narrow, consisting of only 6-10 

 rows of flattened and thick-walled cells. The cells of the spring and 

 summer wood are much larger and rectangular in outline. Their 

 radial diameter is as great as .105 mm. in some cases, while the tan- 

 gential diameter is only .035-. 04 mm. The average size is about 

 .07 mm. in long, and .O3-.O5 mm. in short, diameter. The medullary 

 rays are observed to be numerous. The largest cells are in contact 

 with the medullary rays. 



"Radial. The wood cells or tracheids appear broad and thick-walled, and 

 to be provided with 2 rows of very large pits which nearly touch in 

 the center, and are in contact with the walls on the outside. The 

 diameter of the outer circle is .02 mm., that of the inner circle 

 .oo4-.oo6 mm. They are rarely in a single row when they occupy 

 the center of the cell. The resin ducts consist of a chain of short 

 cells, the contents of which are not preserved. Medullary rays abun- 

 dant ; individual cells long, covering the width of 6 or 8 tracheids; 

 thin-walled. They seem not to have been provided with pits or 

 markings. 



" Tangential. Medullary rays in a single series, and rarely of 1-44 super- 

 imposed cells. It is not common to find rays with less than 5 cells or 

 more than 30, the average being about 10-25. No pits on the walls 

 of the tracheids" (Knowlton). 



Remains silicified. Specimen represented by a log about 30 feet long in 



a clay soil. 

 From the Laramie of Tiger Buttes, Dawson County, Montana (Knowlton). 



19. C. glasgowi, Knowlton 



" Transverse. Annual rings very sharply marked, 3-4! mm. broad. Under 

 the microscope the cells are shown to be arranged in strict radial 

 rows, and in the band of summer wood consist of a layer of 

 18-30 cells more or less completely lignified. In the outer layers 

 of this lignified band of fall wood the lumen of the cells is reduced 

 to a minimum. The lumen is in the form of an ellipse, of which the 

 long diameter is less than .01 mm. and the short diameter about 

 .005 mm. In the immediately following layer of spring wood the 

 cells are very large and thin-walled, measuring .08 mm. in long, and 



