496 
Watson. — On the Structure and 
In a typical specimen of Halonia , preserved so as to retain the leaf- 
bases, such for example as the specimen figured by Williamson in his 
nineteenth memoir, PL VI, Fig. 25 A, as a multiseriate Ulodendron , the 
halonial ‘ tubercles * are seen in the form of large, slightly depressed areas, 
recalling the scars of Ulodendron. They are, however, of very different 
structure ; each scar is divisible into two concentric areas, of which the 
inner is usually smooth and shallowly concave, whilst the outer is divided 
into irregular segments by more or less radial lines. Examination of the 
outer area in any well-preserved specimen will show that it is solely 
impressed on the long leaf-bases of the stem, having been formed by the 
pressure of the equally long leaf-bases of the branch. In the case of any 
of the scars in Williamson’s specimen, the second and third from the top 
in the centre row being the best preserved, the inner area which is not 
leaf-base is 0-5 cm. in diameter, whilst the whole area is 1-5 cm. across. 
These sections should be the diameters of the outer cortex of the branch 
and of the whole branch. A section of Lepidophloios macrophyllum (Q. 428, 
Cash Coll., Manchester Museum), 1*5 cm. in greatest diameter, has an outer 
cortex only 0-5 cm. in diameter, and so agrees exactly with the condition 
which must have occurred in the original of Williamson’s figure. The fact 
that the greater part of the halonial area is made of the leaf-bases shows 
that this type can only occur in Lepidophloios , which genus alone has leaf- 
bases long enough to form the outer part of the area. What is in essence 
this explanation has already been given by Dr. Kidston. 
One very interesting specimen of Halonia from the famous quarry 
‘ Peel Delph which is in the Manchester Museum, is preserved as a very 
fine cast in sandstone of the interior of the outer cortex. This cast is still 
attached to the matrix, and shows that the very prominent tubercles, which 
occur in all Halonias preserved as Knorrias, do not touch the mould of the 
outer surface of the stem by nearly 1 mm. This space must represent 
some tissue in the plant which had not decayed at the time it was buried, 
and as it cuts completely across the base of the branch, this structure can 
only be an abscission layer, similar to that which I have demonstrated in 
structural material. The halonial tubercles themselves obviously represent 
the casts of the holes in the outer cortex left by the decay of the middle 
cortex of the base of the branch. 
The evidence that Dr. Kidston brought forward in the case of the 
halonial branches of Lepidophloios scoticus , although not quite conclusive, 
seems to render it very probable that the appendages of the halonial 
tubercles of that type were cones borne on the end of long peduncles. 
The fact that the possible increase in diameter of lepidodendroid stems 
by secondary growth is very small shows that in such cases as that figured 
by Williamson (nineteenth memoir, PI. VIII, Fig. 23), the lateral appendages 
of halonial tubercles must have been of very large size — at least 2 cm. in 
