XX BRITTRH CxEAPTOLITES. 



" ])ivot " iit one end scorns to iiulicatewitli fair eortaiiity a fixed mode of life. He 

 tlierefove concludes that it is ])rol)al)le that "certain (JraptolitiNs lived attached, 

 while otliei's floated or swam freely in the Silurian seas." 



10. General Gharactos of the Family of the Graptolitcs. — Barrandc points out 

 that since tlie name Graj^folithus was founded on such forms as Gr. Sagittarius and 

 Gr. si'alaris, it should be maintained for all those species showino^ some analogy to 

 these; but ho suggests that the genus GrajdoUthvs shonkl be separated into two 

 sub-genera, viz. Monoprion, including those with one row of colls, and Diprion, 

 embracing those with two parallel rows. 



As regards the curious Y-shaped form figured by Hall as Gr.ramosas, Barrande 

 is inclined to regard it as an accidental phenomenon produced by the splitting of a 

 Diprion form. He considers that the name GraptoUthns should be restricted to those 

 forms having their cells in contact, and he suggests the name of Rastrites for 

 those in which the cells are quite distinct from each other. (This last name has 

 become generally adopted, but the titles Monojjvion and Diprion have never come 

 into general use as generic terms.) Another new generic name proposed by 

 Barrande was that of Gladiolites for a Diprion form covered with a fine network 

 of threads, " and having no axis." His alternative title for this genus, Retiolites, 

 has been more generally adopted. 



11. Distribution and Range of Graptolites in Bohemia. — Barrande deals next 

 with the distribution of Graptolites, first in Bohemia, and afterwards in other parts 

 of the world. In Bohemia the vast majority of Graptolites occur at the l)ase of the 

 (Upper) Silurian; some occur in tho lowest parts of the inferior limestone, Stage E, 

 but they become rarer in tho higher bods, and eventually disappear altogether 

 before Stage F. Regarding the loAvor limits of their range, there is no evidence 

 of their presence in the beds below Stage D, and therefore it is certain that in 

 Bohemia, at any rate, the Graptolites did not make their appearance until after tlie 

 disappearance of the primordial fauna. 



As respects the kind of rock in which Graptolites are found, Barrande calls 

 attention to the fact that they are most abundant in shales, and rare in siliceous 

 rocks; or, as Murchison had previously put it, "the region of Graptolites is a great 

 region of mud." 



Barrande notices especially the occurrence of Graptolites in his " colonies " of 

 Stage D, and indeed employs the Graptolites as evidence of his famous " Theory of 

 Colonies." The existence in the colonies of the same species as are found in the 

 mucli later Stage E, in association with those characteristic of Stage D, seemed to 

 him to afford a proof that the colonial organisms, including the Graptolites, were 

 "derived from a centre of creation (|uito different froin that whei-e the fauna 

 proper to Stage D took its origin ; " and that collectively they wer(> forerunners, 

 so to speak, of a typical Upper Siluriaii fauna, which did not attain its maximum 

 development until a much latci- date. 



