PENGELLY — ON THE DEVONIAN AGE OP THE WORLD. 337 



Amorphozoa and others ; others rich then are poor now, as Brachio- 

 poda and Cephalopoda : whilst Gasteropoda, Lamellibranchiata, &c, 

 abounding in species in existing seas, were formerly by no means 

 thus characterized.* In order to show this numerical relation of 

 the Devonian and existing species, the fifth column of figures, headed 

 Dev. Sjpe., Liv. Spe., in the table, has been calculated thus : — the 

 number of species in each class in the existing fauna, has, for the 

 present purpose, been regarded as normal and put = 1,000, and 

 the number in each of the Devonian classes equated to this ; so that 

 when compared with the specific development of the classes of the 

 present day those of the Devonian age of the world stand, in ascending 

 order, thus : — Cirripedia, Annelida, Pisces, Gasteropoda, Amor- 

 phozoa, Lamellibranchiata, Crustacea, Bryozoa, Echinodermata, 

 Zoophyta, Pteropoda, Cephalopoda, and Brachiopoda. It will be 

 seen also that the number of species in the two last exceeds, and in 

 a high ratio, those of the same classes in existing seas ; whilst those 

 of Gasteropoda and Lamellibranchiata are more than correspondingly 

 abnormally small. Here we have an example of a high class — 

 Cephalapoda — preceding a lower one — Gasteropoda. 



Though when the general fossil census was last taken, the Devonian 

 rocks throughout the world yielded so many as one thousand four 

 hundred and sixty-eight species ;f yet if this number is considered in 

 relation to the great thickness of the deposits of the period, the 

 Devonian strata are poorer in species than either the Carboniferous 

 or Upper and Middle Silurian ; for example, for every one thousand 

 feet in thickness the British Middle Silurian beds contain seventy-nine 

 species ; Upper Silurian ninety-six ; Devonian forty-four ; and Car- 

 boniferous one hundred and twenty. As a rule, deposits charged 

 with peroxide of iron are poor in fossils ; the red limestone of 

 Petit Tor near Torquay, however, is an exception to this, as it is 

 frequently crowded with Orthoceratites and other Cephalopods. 

 It is usual to divide the Devonian system into Lower, Middle, and 

 Upper groups, and this triple division has been applied to Devon 

 and Cornwall, especially by Professor Sedgwick, who recognizes the 

 first, or lowest, in the slates and limestones extending from Plymouth 

 to Torquay, in the limestones of Ilfracombe and Linton, the red 

 sandstones of the north coast, and in the slates of Looe, Polperro, 

 and Fowey, in Cornwall. This he designates the " Plymouth group." 



The middle division consists of the slates extending from Dart- 

 mouth to the metamorphic schists of the Start and Bolt and the 

 slates and purple and greenish sand-rock, stretching in North Devon 

 from Morte Bay, east and west across the country : this is termed 

 the "Dartmouth group," and is probably without fossils. 



The upper includes the rocks ranging from Baggy point by 

 Barnstaple, and the limestone beds and fossiliferous slates of South 

 Petherwin in Cornwall : this is known as the #< Barnstaple or 



* Ibid., 1st and 2nd column of figures, 

 f See " Total" Table 2nd column of figures. 

 VOL. IV. 2 -M 



