GRAPTOLITES. 35 
except in carrying the development upon one side only of the solid axis. 
An illustration of one of these forms is given in fig. 19, plate B, showing 
the base irregularly divided. These forms cannot be referred to any 
known mature species. 
This mode of development, illustrated in numerous specimens, can be 
readily understood in the simple stipes whether of the monoprionidian or 
diprionidian character. Admitting that the examples given furnish 
‘evidence of the mode of reproduction of the diprionidian forms, or those 
of the sub-genus Diplograptus, where we have a range of cellules on each 
side of a solid axis, it is easy to perceive how the germ of an analogous 
form may develop from its initial point two series of cellules upon a stipe, 
where the parts diverge im opposite directions from the common origin or 
initial pot. One step farther in this direction will give us the four-stiped 
forms, where the germ of the common body, with its additional elements 
of subdivision, produces the quadripartite frond; and so onwards, until 
we have the numerously-branched fronds and the branching stipes. 
Tn all these, the germ in its incipient development will differ very little. 
It may consist of the radicle or initial point, with the solid axis, and the 
common body, separated into two, four, eight, or an indefinite number of 
divisions, each one bearing its solid axis and common canal. These sub- 
divisions sometimes all take place near the origin, which is always central ; 
and the divisions continue simple throughout, or do not bifurcate after 
they commence to develop cellules. 
In others the stipes are again divided, and this subdivision is only 
limited by the extent of the frond. In all these fronds the parts are always 
arranged symmetrically or bilaterally on the two sides of the initial point ; 
as has been illustrated in the preceding pages. 
Throughout all the monoprionidian forms, or those illustrated on the 
first twelve plates of this memoir, we have only modifications of the 
simplest form of development shown in the species of plates i and ii. Where 
the divisions at the base become more numerous, (and indeed in the four- 
stiped species, ) we find a thick corneous test, of the same substance as the 
other parts of the graptolite, uniting the bases of the stipes and continuing 
along their margins. This plate has a greater or less development, not 
always corresponding to the size or extent of the stipes. It is sometimes 
absent, apparently from accident, and some of the four-stiped species are 
not known to possess it; while it has never been observed in any of the 
species where the stipes are properly branched, or divided in the celluli- 
ferous parts of their length. 
The interior of this corneous disc, previously described as apparently 
composed of two plates of the test, has probably been occupied by some 
softer substance, which may have been an extension of the common body, or 
have had in some degree the character of the common body of the stipe. 
