Holden . — Jurassic Coniferous Woods from Yorkshire . 543 
Conclusions. 
There are several interesting conclusions to be drawn from these 
Jurassic woods of Yorkshire. The most important is the bearing of such 
transitional forms on the relative antiquity of the Abietineae and Araucari- 
neae. From a study of the comparative anatomy of living representatives, 
Jeffrey (7) seems to have demonstrated beyond logical doubt that the 
Araucarineae are descended from ancestral forms which had, in the mature 
wood, resin canals, heavily pitted rays, wood parenchyma, and Brachyoxylon 
pitting. Such hypothetical ancestors are materialized in Metacedroxylon , 
P aracnpressinoxylon , Paracedroxyl on , &c. Fossil evidence is not of course 
always perfectly clear, for in many cases the Abietineous features of an 
Araucarian Conifer may, with equal reason, be interpreted either as persist- 
ing from a typical Abietineous form which is on the reduction path to the 
Araucarineae, or as incipient in an Araucarian which is on the upward way 
to the Abietineae. On the other hand, whenever experimental evidence is 
available for the study of extinct forms, such ambiguity is not possible. 
Thus, when these fossil Araucarian Conifers, on wounding, give rise to 
traumatic resin canals it seems evident that in its primitive condition 
the Araucarian stock had resin canals normally, and that the series is 
a reduction one from Abietineae to Araucarineae. There is, however, 
no possibility of a double interpretation in the evidence furnished by those 
Conifers existing at the present day. Under these circumstances the 
explanation which is universally applicable — both to living and extinct 
forms — appears beyond doubt the correct one. 
Another fact leading to the same conclusion may be obtained by com- 
paring the transitional forms of different geological horizons. In the 
Cretaceous the majority of these intermediate forms were of the Brachyo- 
xylon type, with thin-walled rays, and a mixture of Araucarian and 
Abietineous pitting. In the Jurassic, on the other hand, as shown by the 
investigations of Gothan, Seward, Lignier, and others, the majority had 
thick-walled pitted rays, with a distinctly less Araucarian type of tracheary 
pitting. In other words, the farther back geologically we go, the more like 
the Abietineae do the transitional forms become. 
If it be granted that the Araucarineae were derived from the Abietineous 
Conifers which were characterized by the possession of resin canals, wood 
parenchyma, and thick-walled rays, it is easy to understand how, in the 
course of reduction, different combinations of these features should be found 
in early Araucarians. Thus Protopiceoxylon , Gothan, is probably an 
Araucarian Conifer which has not yet lost its resin canals ; while Para - 
cupressinoxylon retains its wood parenchyma, and Metacedroxylon its pitted 
rays. Whether P araphyllocladoxylon should be interpreted as a Phyllo- 
