GRAPTOLITES. 39 
tive sac, are furnished with the minute radicle-like appendage or extension 
of the solid axis, as well as the oblique lateral processes like tentacula ; 
and the condition of these parts does not seem to have been essentially 
changed during any subsequent period of their growth. While the 
extension of this slender solid axis does not seem of sufficient strength to 
have formed the base of attachment to the sea-bottom, it may have been 
sufficient to maintain connection with other parts of a compound frond. 
For all those species with a single range of cellules, as well as for some 
with a double range, including Retiohtes, Retiograptus, and Phyllograptus, 
I conceive that we have already shown a similar plan of development and 
a uniform mode of existence; and we are constrained to believe that all 
these forms, in their mature condition, were free floating bodies in the 
Silurian seas. 
In regard to another group including Dendrograptus, Callograptus, 
and Dictyonema, as well as one or two other forms, we have some evidence 
indicative of a different mode of existence. The stems of Dendrograptus 
are enlarged towards their base, and sometimes present a sudden expansion 
or bulb, which I have inferred may be the base or root, once attached to 
another substance or imbedded in the mud. The general form of the 
species conduces to the belief that they were fixed to the sea-bottom, 
though possibly this basal expansion may have resembled that of Graptol- 
thus bicornis. In most of the species described, the lower a is 
imperfect, and its termination unknown. 
Tn those which I have termed Callograptus, the bases of the fronds are 
imperfect, but indicate, according to analogy, a radicle or point of attach- 
ment like Dendrograptus. In the more nearly entire forms of Dictyonema 
known, we have not been able to observe the base; but from their simi- 
larity in form and mode of growth to Fenestella and Retepora, we have 
inferred their attachment either to the sea-bottom or to foreign bodies. 
Nearly all these forms occur in rocks where there are few of the larger 
fossils of any kind except the graptolites; so that there is little chance 
of finding their bases attached to shells and corals, as we do those of the 
bryozoans, even if they had thus existed, The Dictyoneme of the 
Niagara, Upper Helderberg, and. Hamilton groups do occur in strata which 
contain large numbers of other fossils ;. but we have no evidence of their 
having been attached. It is only from their general form therefore, and 
from their analogy with other bodies, that we infer that these genera may 
have been attached to the sea-bottom or to some objects during their 
growth. 
_ We admit therefore that the family of Graptolitidz, as now dstertioal 
may include both free and fixed forms. 
