764 PROFESSOR W. C. WILLIAMSON AND DR. D. H. SCOTT ON THE 
H. tilieoides appears to us to agree in all essential points of structure with 
H. Grievii and certainly to be rightly placed in the same genus. It differs from that 
species in the exceptional development of its secondary phloém and phloém-rays, in 
the greater distinctness of the primary bundles at the periphery of the stele, in 
the consequent separation of the secondary tissues into distinct strands, in the 
presence of sclerotic elements in the pericycle, and in the double leaf-trace bundles. In 
several of these points the present species approaches Lyginodendron somewhat more 
nearly than H. Grievit does. On the whole, we must regard H. teleoides as the 
more highly differentiated of the two Heterangiums. Its great interest to the 
botanist depends chiefly on the extraordinary perfection with which the details of its 
structure are preserved, affording the clearest proof, that in the carboniferous flora, 
the histological elements were identical with those of existing plants, however much 
the arrangement of these elements may have differed. 
iii, On A Heterangium or UNCERTAIN SPECIES. 
We have a single specimen of the stem of a Heterangiwm, probably from Halifax, 
which we are not able to refer with certainty to either of the preceding species. A 
transverse section of this specimen is shown in fig. 30, and a part of the longitudinal 
section in fig. 3]. The general structure of the stele agrees with that of the other 
two species. The appearance of the transverse section rather suggests H. telaeoides 
than H. Grieviz. The presence of sclerotic groups in the pericycle is a definite 
character in which it resembles the former species. The longitudinal section, however, 
shows that the primary tracheides exactly resemble those of H. Grievi, while they 
are somewhat different from those of H. tiliwoides. In the latter many of the 
primary tracheides show curious spiral lines of thickening between the series of pits. 
Nothing of the kind is visible in H. G'rieviz or in the undetermined specimen. Only 
a very slight formation of secondary wood has taken place in this specimen. The 
cortex is of moderate thickness and has a smooth outer surface, so that the stem, as 
a whole, has a cylindrical form, and is thus very different from the conspicuously 
ribbed stem of H. Grievi. The comparison with the form of the stem in H. tilieoides 
is difficult, as the cortex is never complete in the specimens of the latter species. 
The inner cortex contains masses of dark sclerotic tissue, arranged in vertical series 
(see fig. 31), while the outer zone consists chiefly of sclerenchymatous fibres. There 
is thus nothing peculiar in the structure of the cortex ; its appearance is somewhat 
unusual and suggestive of charring. The transverse section shows a fairly-preserved 
pair of leaf-trace bundles (fig. 30, /t.). The fact of their forming a pair suggests a 
comparison with H. tzli@oides, though this arrangement is not absolutely unknown in 
the other species. 
A tetrarch root of the type which we have referred to Heterangium, lies close to 
the stem opposite an interruption in the cortex. It is not unlikely that it may have 
