8 4 
Seward. — A Contribution to our 
structure and form of the medullary rays, usually enable us 
to refer a specimen to a definite family or group of plants. 
In dealing with coniferous wood we are able to adopt certain 
recognized generic designations, such as Araucarioxylon , Pity- 
oxylon , and others, which, as somewhat comprehensive terms, 
are extremely useful in systematic work. The term Cyca- 
doxylon proposed by Renault, if extended in its application, 
might serve as a general generic designation for plants 
possessing secondary xylem closely resembling that of recent 
Cycads. Such a term would be particularly useful in dealing 
with imperfect material ; while the more complete data 
obtained from better specimens would enable us to make 
use of generic designations of a less comprehensive meaning. 
In his important Memoir on the comparative structure of 
certain Coal-Measure stems, Renault includes the three 
genera Cycadoxylon , Colpoxylon , and Medullosa in the group 
Cycadoxyleae ; and as a characteristic of the group he 
mentions that the genera in question have lost the centri- 
petal wood in their stems, while retaining it in their leaves b 
In the more recent; and fuller, account of Cycadoxylon , 
Renault figures and describes well-marked bands of cen- 
tripetal wood, thus necessitating either a modification of 
the original definition of the Cycadoxyleae, or a removal 
of Cycadoxylon from that group of plants in which the stem 
has no internal centripetal wood. 
If Cycadoxylon were adopted in a wider sense than as 
defined by Renault, it would be the more appropriate genus 
to which to refer Lyginodendron anomalum. In the case of 
Lyginodendron robustum alone it might perhaps be better to 
make use of this 'generic designation; but the presumptive 
evidence in favour of a generic identity with Lyginodendron 
Oldhamium is so strong, that there appear to be sufficient 
grounds for the choice of the more restricted genus Lygino- 
dendron. Without attempting to institute a comparison with 
the various Palaeozoic genera possessing cycadean characters, 
1 Nouv. Arch. Museum, Paris, 1879, p. 282. 
