Distribution of the Rliabdopliora* 193 



and an Upper or Upper-Ludlow-Downton (Downtonian) 

 division. 



Subordinate to these are the formations and subformations 

 generally recognized among geologists. 



Placing these zoological and geological scales in juxtaposi- 

 tion (see Table XL), we recognize immediately that most 

 intimate correspondence between life-type and time-epoch 

 which is inevitable upon any theory of gradual evolution. 

 Our grandest zoological groups do not, it is true, fit in indi- 

 vidually with the several rock-systems ; but the time-ranges 

 of our chief generic types admit of rigid localization and 

 admeasurement upon our geological scale, and form a series 

 which may be paralleled with that of our recognized subfor- 

 mations with wonderful accuracy. On a general review of 

 all these correspondences, as detailed in the foregoing pages, 

 it is evident that they establish the following propositions : — 



(i.) The Rhabdophora, or true Graptolites, are exclusively 

 Lower-Palaeozoic fossils, coming into visible existence in the 

 Upper Cambrian, and disappearing from sight in the Upper 

 Silurian. 



(ii.) They attain their maximum, both in genera and species, 

 about the middle of this range, i. e. in the Llandeilo formation ; 

 and there is a gradual decrease in forms in proportion as we 

 pass upwards or downwards from this horizon. 



(iii.) The three grand groups of the Didymograpta, Dicello- 

 grapta, and Monograpta are so restricted in their vertical 

 range that each distinguishes a certain portion of the ascending 

 succession of formations. The Didymograpta are essentially 

 Lower-Ordovician fossils, the Dicellograpta Upper-Ordovician, 

 while the Monograpta are confined exclusively to the Silurian 

 proper. 



(iv.) With but two exceptions, each of the families of the 

 Rhabdopora ranges through a fraction only of the entire suc- 

 cession of the Lower Palteozoic rocks, nowhere exceeding in 

 vertical extent that of an entire system. The Dichograptida? 

 are Upper-Cambrian and Lower-Ordovician fossils ; the 

 Phyllograptidse are exclusively Arenig; the Leptograptida3 

 and DicranograptidEe are essentially Upper-Ordovician ; while 

 the Lasiograptidee are as rigidly confined to the Ordovician 

 itself as the Monograptidae are to the succeeding Silurian. 



(v.) Among the genera this limitation in time is carried out 

 even more minutely. Loganograptus, Tetragraptus, Dicho- 

 graptus, Retiograptus, and several others are exclusively 

 Arenig genera. Pleurograptus, Amphigraptus } Coenograptus, 

 &c. are peculiar to the Bala. Rastrites distinguishes the Valen- 

 tian, and Cyrtograptus the Salopian. 



