234 RETROSPECT. 



peculiar families belonging exclusively to the early Palaeozoic 

 period such as the Cephalaspides, which possessed great bony 

 plates, some like a helmet covering the head, and others like a 

 shield protecting the body. 



Hitherto land-animals have not been found in the early 

 Palaeozoic rocks ; and land-plants make their first appearance in 

 the Devonian formation. But few species, however, are known ; 

 and these are most nearly allied to plants of the Carboniferous 

 period. Monocotyledons and Dicotyledons are entirely wanting ; 

 and there are only a few Gymnosperms, which differ so widely 

 from all existing forms that there is great doubt respecting their 

 place in the botanical system. 



Most of the plants of the early Palaeozoic rocks belonged to 

 the vascular cryptogams ; and, as in the Carboniferous period, 

 Ferns, Lycopodiaceae, and Calamariae constituted the vegetation 

 which covered the green islands of the primaeval sea. Among 

 the Ferns, species of the genera Sphenopteris , Cyclopteris, and 

 Odontopteris are met with ; the Lycopods are represented by a 

 Lepidodendron, and the Calamariae by a number of peculiar 

 types of which Unger has made genera and even distinct fami- 

 lies ; but as this determination is founded only upon a few re- 

 mains of stems, Prof. Heer can form no distinct conception of 

 their aspect. 



As the early Palaeozoic strata do not occur in Switzerland, 

 or at least as no certain demonstration can be given of their 

 existence in that country "*, an intermediate place will be given 

 to them between the Primary rocks and those of the Carboni- 

 ferous period in the following Table (p. 235) of the chief periods 

 in the history of the earth's crust. 



If the deposition of the rock -strata had taken place from the 

 commencement quietly and without disturbance, the evidences 

 of these different periods would be found one above the other in 



* Prof. Studer (Geologie der Schweiz, i. p. 346) thinks that the grey 

 schists which surround the eastern central mountain-masses of the Vorarlberg, 

 and traverse the Grisons as far as Ortles, may belong to the Silurian and 

 Devonian formations, as rocks containing Silurian and Devonian animal- 

 remains are found under similar conditions in Styria. 



