GRAPTOLITOIDEA. 



221 



of the Ordovician and Silurian deposits, as has been amply demon- 

 strated by the researches of Lapworth, Linnarsson, and others. 

 Regarded as a whole, the Graptolites, as previously stated, are 

 restricted to the Upper Cambrian, Ordovician, and Silurian rocks, 

 but particular types have been shown to be characteristic of particu- 

 lar horizons in these rock-systems. This is not the place to give an 

 account of the general succession of the Graptolitoidea in time, or to 

 indicate the precise stratigraphical value of particular forms of the 

 group, but some general results may be briefly alluded to. The 

 great family of the Monograptidce is wholly confined to the Silurian 

 rocks proper (Upper Silurian of Murchison), the genus Monograptus 



A 



Fig. 106. — Different views of portions 

 of Retiolltes venosus, enlarged nine 

 times, showing the netted periderm and 

 the virgula. (After Hall.) 





Fig. 107. — Group of individuals of Phyllo- 

 graptus typus, from the Quebec group of 

 Canada (after Hall). One of the four rows 

 of cells is hidden on the under surface. 



itself extending from the base of this system to its summit. On the 

 other hand, the families of the Diplograptidcz and Retiolitidcz range 

 from the base of the Ordovician rocks to the middle of the Silurian. 

 The families of the Leptograptidce, Dicraiiograptidce, Lasiograptidce, 

 and Phyllograptidce are restricted to the Ordovician rocks, and the 

 species of Phyllogr aphis, the sole representative of the last of these 

 families, is only found in the basal beds (Arenig and Llanvirn beds) 

 of the Ordovician. Lastly, the great family of the Dichograptidce 

 commences in the Upper Cambrian period, culminates in the lower 

 Ordovician, and dies out altogether before the commencement of the 

 Silurian. The horizon of the Arenig rocks (Skiddaw and Quebec 



