238 REPORT ON THE STATE CABINET. 



The expansion at the base of this species has the same general appear- 

 ance as the central disc of G, logani, G. headi, and others ; showing that 

 this sort of development of the substance is not alone characteristic of 

 those forms having several stipes united at the base. In other examples 

 this basal expansion is contracted in such a manner as to give a crescent 

 form to the lower extremity ; but in all these gradations the margins ot 

 this part are entire and unbroken. 



We have seen that the youngest forms of the diprionidian Graptolites, 

 those which we may suppose had but recently escaped from the repro- 

 ductive sac, are furnished with the minute radicle-like appendage or 

 extension of the solid axis, as well as the oblique lateral setiform pro- 

 cesses ; 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 

 polypary ; but this condition has never been proved true of any of the 

 species of Diplograptus proper. 



For all those species with a single range of cellules, where the stipes 

 are combined in a compound form, as well as for some with a double range, 

 including Eetiolites, 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 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 difierent 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 or sand of 

 the sea-bottom. 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 bircornis. In most of 

 the species described there is a gradual enlargement towards the lower 

 extremity, which 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 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- 



