STUDY OF THE GBAPT0LITE8. 239 



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 had they thus existed. The 

 DiCTYONEMA of the Niagara, Upper Helderberg and Hamilton groups do 

 occur in strata which contain other fossils ; but we have no evidence 

 of their having been attached to any of these. 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 Graptolitidag, as now extended, 

 may include both free and fixed forms. 



VII. General Characters of the Family GraptoUtldce ; With Reference to 

 the Distinctive Features of the Genera, as Known in the Geological Form- 

 ations of 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 those 

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



The numerous Graptolites described by Nilsson, Hisinger, Bronn, 



MURCHISON, ElCHWALD, PoETLOCK, GeINITZ, BaRRANDE, SuESS, McCoY, 



Salter, Hajikness, Nicol, Meneghini, Nicholson,* Carruthers,* myself, 

 and others, are for the most part in a fragmentary condition, affording 

 knowledge only of the simple stipe, the structure of its parts, and the 

 arrangement of the cellules. From these fragments, however, we have 

 derived the generic characters; while the modifications in form, and the 

 order and relations of cellules, have furnished means of specific distinc- 

 tions in the greater proportion of those described. 



In maintaining the generic term Graptolithus for the forms which 

 have the nearest relations with those to which the term was originally 

 applied by Linn^us, M. Barrande has proposed two sub-genera, charac- 

 terized by the presence of a single series, or of two parallel series of 

 cellules, under the name of Monoprion and Diprion. The latter term 



♦Since the original publication of this memoir. 



