[PENHALLOW] NORTH AMERICAN SPECIES OF DADOXYLON 59 
The peculiar distribution of the bordered pits constitutes one of 
the most prominent and distinguishing features of the genus, and dis- 
tinctly allies it to the Araucarieæ. Unger lays down as a generic char- 
acter, “Vasa porosa, poris in series 1—4 spiraliter dispositis,”* which is 
simply a restatement of Brongniart’s recognition that “ les ponctuations 
des fibres ligneuses disposées en plusieurs séries alternantes entre elle, 
et prenant par pression la forme d’aréoles hexagonales.” All studies 
show that the pits are distributed throughout the length of the 
tracheids, as well as that they regularly occur on the radial walls of all 
tracheids, thus presenting an important contrast to the law of distribu- 
tion which excludes them more or less fully from the tracheids of the 
summer wood of the Abietineæ. 
While these laws of distribution are well recognized as applicable 
in a generic sense, the various recognised species show great diversity 
in this respect, and such variations have been taken as an element in 
diagnosis. The possible value of such data is fully exemplified in the 
case of C. Newberryi, where the pits occur in well defined groups which 
fall into radial rows. This feature has unfortunately been neglected 
entirely, by every writer except Prof. Newberry himself, but its im- 
portance as a differential character cannot be doubted (fig. 12). It is 
nevertheless a well recognised fact that the number of rows in which 
pits occur is such an altogether variable factor within the same in- 
dividual, as to render it untrustworthy as a differential character except 
when employed with great caution.* Under these circumstances we can 
only consider such data as of service in so far as they help to reinforce 
other characters indicative of specific differences, while at the same time 
they tend to enforce the idea that one should be very cautious in 
creating new species unless such can be justified by well defined 
characters. 
The medullary rays require to be discussed from two points of 
view. The tangential exposure shows the cells to be disposed in one 
row, becoming 2-seriate in part, and Kraus has employed this feature 
as a means of differentiation from those other genera in which multi- 
seriate rays occur.* In a radial section the rays are seen to be composed 
of one kind of element only, with the exception of C. Clarkii, in which 
definite tracheids are to be found in addition to the ordinary parenchy- 
matous elements. This is the only species within my observation, which 

1 Gen. et Spec. Pl. Foss., 1850, 378. 
* H. von Mohl, Bot. Zeit., 1862, 225; Kraus, Wurzburger Naturwiss. Zeit., 
V., 1864, 185. 
#5 Schimper, Pal. Veg., 379. 
