xcvi BRITISH OxRAPTOLITES. 



graphically and morphologically the Graptolites arrange themselves into four 

 groups : 



(1) Monograpta (including the family of the Monograptidae only). 



(2) Diplograpta (including the families of the Diplograptidae, Lasiograptidae, 

 and Retiolitidae). 



(3) Didymograpta (including the Dichograptidae and Phyllograptidas). 



(4) Dicellograpta (including the Dicranograptidae and Leptograptidae). 



He gives a table showing the Vertical Distribution of the component genera, 

 and lie considers the following propositions as established : 



(1) " 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." 



(2) "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." 



(3) " The three grand groups of the Didymograpta, Dicellograpta, and Mono- 

 grapta are so restricted in their vertical range that each distinguishes a certain 

 portion of the ascending succession of formations. The Didymograpta are essenti- 

 ally Lower Ordovician fossils, the Dicellograpta Upper Ordovician, while the 

 Monograpta are confined exclusively to the Silurian proper." 



(4) " With but two exceptions, each of the families of the Rhabdophora ranges 

 through a fraction only of the entire succession of the Lower Palaeozoic rocks, 

 nowhere exceeding in vertical extent that of an entire system. The Dichograp- 

 tidae are Upper Cambrian and Lower Ordovician fossils; the Phyllograptidae are 

 exclusively Arenig ; the Leptograptidae and Dicranograptidae are essentially Upper 

 Ordovician; while the Lasiograptidae are as rigidly confined to the Ordovician 

 itself as the Monograptidse are to the succeeding Silurian." 



(5) " Among the genera this limitation in time is carried out even more 

 minutely. Loganograptus, Tetragraptiis, Dichograptus, Retiograptus, and several 

 others are exclusively Arenig genera. Pleurograptus, Amphigraptus, Oceiiograptus, 

 etc., are peculiar to the Bala. Rastrites distinguishes the Valentian, and 

 Cyrtograptus the Salopian." 



((')) " Descending to the species of the Rhabdophora, we find that they arc so 

 restricted in vertical distribution ( hat few have a more extended range than that 

 which is covered by a single formation in the vertical series; while the vast 

 majority are peculiar to a single sub-formation, or marl? certain special horizons 

 outside of which they are unknown. The forms which have the greatest 

 longevity present us with the greatest number of recognisable varieties, while the 

 species of shorter range rarely show any notable departure from the primitive 

 type." 



(7) "The ascertained restriction of the divisions, families, and genera of the 



