STUDY OF THE GRAPTOLITES. 207 



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, Callograpttts, 

 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 Graptoli- 

 thus hicornis. 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 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, and indeed few 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 

 DictyonemcB 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 tlieir 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 other 

 objects during their growth. 



We admit therefore that the family of Graptolitidse, as now extended, 

 may include both free and fixed forms. 



VII. — GENERAL CHARACTERS OF THE FAMILY GRAPTOLITID^ ; WITH 

 REFERENCE TO THE DISTINCTIVE FEATURES OF THE GENERA, AS 

 KNOWN IN THE GEOLOGICAL FORMATIONS OP CANADA AND THE 

 UNITED STATES. 



In the first section of this memoir, I have remarked upon the nature 

 and general form of the graptolites proper, and the allied genera which I 

 regard as belonging to the same family. The large accession to the 

 number of species, and the great variety of new forms added to tliose 

 formerly known, require an extension of the characters heretofore given. 



The numerous graptolites described by NiLSSON, Hisinger, Bronx, 



MURCHISON, ElCHWALD, PoRTLOCK, GeiNITZ, BaRRANDE, SuESS, 



McCoy, Salter, Harkness, Nicol, Meneghini, myself, and other/?, 

 are for the most part in a fragmentary condition, affording' knowledge 

 only of the simple stipe, the struclure of its parts, and the arrangement of 



