[PENHALLOW] NORTH AMERICAN SPECIES OF DADOX YLON 57 
most important diagnostic feature. Of the eighteen species now en- 
titled to recognition, three show more or less clearly defined growth 
rings, while in the remaining fifteen, they are obscure or obsolete. It 
therefore appears that there are fifteen species of Cordaites which con- 
form to the general type of Araucaria, while three conform to the type 
of Dammara. This is in strict harmony with the generally accepted 
view that these plants are of distinctly Araucarian affinities but without 
implying actual connection. Inasmuch, however, as the growth rings 
are initiated by alternating growth and rest periods which are in turn 
determined chiefly by climatic conditions, such structural variations are 
not necessarily indications of relationship, and their chief importance 
may lie in their indication of varying climatic conditions as operating 
upon different species of the same genus. Interpreting these considera- 
tions in the narrow sense, it would seem as if the genus Cordaites must 
have included species which flourished under widely different climatic 
conditions; but interpreted in the broader sense of the Araucarieæ as 
a whole, we are to conclude that the genus as now constituted, embraces 
in reality what must eventually prove to be distinct genera of the separ- 
ate types of Araucaria and Dammara as parallel examples among existing 
Araucarie. ; 
All observations agree in the fact of a complete absence of resin 
canals such as occur in the Abietinez on the one hand, and of specialised 
resin cells such as occur in the Cupressineæ on the other hand, and in 
this we observe further agreement with the modern Araucarieæ, but as 
in the latter, all known species show undoubted evidence of the more 
or less abundant occurrence of resin in the medullary rays, and also in 
the tracheids of the woody cylinder. The principal occurrence of this 
material, however, is in the pith, where it is deposited in cells (fig. 8, 
r. c.), or in the bark where it is developed in connection with specialised 
resin canals (figs. 2, 6, 7, r. c.) or in special cells. In these respects there 
is a somewhat close resemblance to what may be noted in Araucaria. 
The bordered pits constitute one of the most prominent and char- 
acteristic structural features of these plants. Some of the species under 
examination show in a somewhat remarkable manner, the gradual trans- 
formation of the scalariform structure into that of the typically bordered 
pit. and in no case is this exhibited with greater clearness than in C. 
Brandlingii (figs. 9, 10, 11), where a properly selected radial section will 
show all the gradations from one extreme to the other. This fact is by 
no means a new one, since so long ago as 1840, Don pointed out that 
tracheids of Cycas revoluta would exhibit scalariform structure at one 
