534 Holden . — Jurassic Coniferous Woods from Yorkshire . 
the Abietineae. In this type, which he called Br achy oxy Ion , some of the 
pits are closely compressed and flattened, while others are scattered and 
circular. Further (7), he has shown that in the seedling of both living 
Araucarian genera — Araucaria and Agathis — this Brachyoxylon type 
persists. Since there is this marked departure from the so-called Arau- 
carian pitting even in living forms, it seems to follow that this type of 
pitting will be even less constant in forms which are now extinct. Accord- 
ingly it appears to be unsafe to employ this criterion exclusively in diagnosing 
fossil woods. 
Gothan also considers of little importance the character of the 
tracheary pitting, for he describes as Abietineous a number of fossils with 
typical Araucarian pitting. His criterion is the nature of the medullary 
ray cells — any Conifer whose rays have thick, heavily pitted walls is 
Abietineous, no matter what its other structural peculiarities may be. 
Neither Lignier nor Jeffrey follows him here, for both have described as 
Araucarian specimens with pitted rays — Cormaraucarioxylon crasseradia- 
tum , Lignier (op. cit.), and Araucariopitys americana , Jeffrey (8). Moreover, 
the last-mentioned writer has shown that in the cone axis of Agathis australis , 
and in traumatic wood of both Araucaria and Agathis , there are typical 
Abietineous rays. It seems evident that in living forms the character of 
the rays is as inconstant as that of the pitting of the tracheides, and is 
accordingly as little to be relied on in fossil forms. 
The only feature which holds absolutely is the occurrence of certain 
cellulose 1 thickenings, embedded between the pits in the substance of the 
cell wall itself. These are the so-called ‘ bars of Sanio ’, which are present 
in all woods of Abietineous affinities — Abietineae, Cupressineae, Taxodineae, 
Podocarpineae, and Taxineae (9) — but are invariably absent in those of 
Araucarian affinities, except in the first few secondary tracheides of the cone 
axis of Ara 2 ic aria and Agathis (7). Since this is the only character which 
is perfectly constant in those Conifers existing at the present day, it appears 
to be the safest to choose in diagnosing fossil forms. The most striking 
case where this feature alone has been considered sufficient to decide the 
affinities of a wood is that of Paracedroxylon , Sinnott (10), which its founder 
calls Araucarian in spite of scattered pits, resin canals, and thick, pitted rays 
(both these last two features, however, are traumatic). It has been suggested 
that ‘ bars of Sanio ’ are structures too delicate to be preserved in petrified 
material. Such is not the case, for in many silicified specimens they are 
as unmistakable as in living forms. Owing to the fact that they are com- 
posed of cellulose, which speedily disappears in the course of fossilization, 
1 By cellulose is meant giving the ordinary cellulose reactions, e. g. a dark blue stain with 
iodine and sulphuric acid. Whether they are called 1 bars of Sanio ’, or, as Professor Groom prefers, 
‘ rims of Sanio,’ is a matter of absolutely no importance as long as it is understood that they contain 
cellulose, and that they are embedded in the cell wall. 
