52 THE CANADIAN NATURALIST [Feb. 



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 sub- 

 sequent period of their growth. While the extension of this slen- 

 der solid axis does not seem to have 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 com- 

 pound frond. 



For all those species with a single range of cellules, as well as 

 for some with a double range, including Retiolites, 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, Callo- 

 graptus, 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 sub- 

 stance 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 

 Graptolithus bicornis. In most of the species described, the lower 

 extremity is imperfect, and its termination unknown. 



In those which I have termed Callograptus, the bases of the 

 fronds are imperfect, but indicate, according to analogy, a radicle 

 or point of attachment like Dendrograptus. In the more nearly 

 entire forms of Dictyonema known, we have not been able to 

 observe the base ; but from their similarity in form and mode of 

 growth to Fenestella and R'depora, we have inferred their attach- 

 ment 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 

 Dictyonemce 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 



