488 
PROFESSOR W. C. WILLIAMSON ON THE ORGANIZATION 
been channels for the transmission of special vessels, may have been of this variety. We 
have only seen two stems of this type*. Plate XXV. fig. 17 represents a transverse 
section of the half of one of the primary medullary rays (c), and the adjacent part of a 
woody wedge (f) of a remarkable form. Fig. 18 is a carefully drawn copy of a tangential 
section of part of a similar primary medullary ray from the same example. The laminee 
of vascular tissue (g), instead of terminating at the usual boundaries of the woody wedge 
( /), have continued to be developed externally to it, encroaching upon the primary medul- 
lary ray, as exogenous growths increased the diameter of the stem. Practically the 
result of these additions has been to convert each large primary medullary ray into a 
series of smaller secondary ones, with but one, two, or three linear rows of cells in each, 
reducing them to a condition differing little, in the transverse section, from what we find 
in the secondary medullary rays of the woody wedges. But when we turn to the tan- 
gential section (fig. 18), we discover that these vessels ( g ), not having been subjected 
during growth to the uniform pressure mutually affecting those in the woody wedges, 
have not only pursued a more tortuous course amongst the large cells, but their form 
and diameter has been modified by the unequal resistance of those cells, so that whilst 
in some instances their diameter is ’005, in others they are reduced to a mere thread, 
I have only met with one example of this remarkable variety. 
I have next to call attention to a peculiar form identical in many respects with one that 
I described in the fourth volume of the third series of the Memoirs of the Literary and 
Philosophical Society of Manchester, and to which I gave the generic name of Calamo- 
pitus. Some examples of this type possess the highest interest, because they throw a 
most important light on the nature of the forms of Calamite so common in the shales 
and sandstones of the Upper Coal-measures. Fig. 19 is a representation, of the natural 
size, of a transverse section of a compressed stem. The dark centre has been a large 
fistular medullary cavity, whilst the walls of the surrounding cylinder have been remark- 
ably thin in proportion to the size of the stem ; the diameter of the latter has been about 
•66, the entire thickness of the decorticate woody cylinder, including its contained layer 
of pith-cells, not having been more than '08 — a condition of things most favourable to 
that flattening of the stem so frequently seen in the fossils which are laid horizontally 
in their matrix. Though this cylinder is so thin, it contains about eighty distinct woody 
wedges. Plate XXY. fig. 20 represents two of these woody wedges more highly mag- 
nified. Two features alone require to be noted in this section. One is the well-marked 
crenulated outline (x, x ) separating the pith from the persistent woody zone. This line is 
especially remarkable for its distinctness where it crosses the primary medullary rays ( c ). 
In this feature the specimen resembles my previously described Calamopitus, as well as 
fig. 15 of the present memoir; only its crenulations are much deeper, approaching less 
towards a straight line, than in the latter figure. The second point referred to is the 
remarkable prominence exhibited by the external base of each woody wedge (f), which is 
* Since the above description was written I have met with a third specimen, and think it possible that a 
new genus may be required for its reception, since it lacks some Calamitean features. 
