NATURAL HISTORY OF THE PRIMEVAL WORLD. 



The mouth was- placed forward and beneath the 

 head. No antennae or limbs have been recognized in 

 any species ; and it is probable their feet were simply 

 soft, leaf-like appendages, carrying the gills, which, from 

 their perishable nature, would leave no traces in a fossil 

 condition. 



The Trilobites varied greatly in size. While many 

 were no larger than a pin's head, others as the 

 Asaphus gigas were eighteen inches long. Upwards 

 of 400 species have been determined. Of these forty- 

 six are Silurian. They wholly ceased to exist after the 

 Carboniferous epoch. Their structure is a beautiful 

 example of the articulated type, and admirably illus- 

 trates that unity of design which, is conspicuous through 

 all the diversity of nature. 



Many other orders of molluscs were abundantly 

 represented in the primeval ocean of the Silurian period. 

 Among the Cephalopoda we may name the Gyroceras 

 and Lituites cornu-arietis, or ram's horn, which, like 



Lituites corrm-arietis. One-third natural size. 



the Orthoceras, were allied in many respects to our 

 modern Sepia and Nautilus. The Gasteropoda (belly- 

 footed), so called because their locomotive apparatus is 

 attached to the under part of the body, found a type in 

 the genus Bellerophon, which, with their elongated 

 transparent body and rudder-like fin, resembled the 

 existing Carinaria, and inhabited the lowest depths of 

 the sea. Several species of Lamellibranchiata (an 

 order of Acephalous or headless molluscs) also existed : 

 in these the shell consisted of particles of carbonate 

 of lime, exuded from the surface of the mantle, and 



whose nature is so fully disclosed by the state of perfection in 

 which they are preserved. With regard to the atmosphere of 

 the then existing world, we may also infer that had it differed 

 materially from its actual condition, it might have so far affected 

 the rays of light, that a corresponding difference from the eyes 

 of existing crustaceans would have been found in the organs on 

 which the impressions of such rays were then received. 



We may also learn, respecting light, from the resemblance of 

 these most ancient organizations to existing eyes, that the mutual 

 relations formerly existing between the eye and light were the 

 same at the time when crustaceans gifted with the faculty of vision 

 lirst flourished in the primeval seas, as at the present moment. 



Hence it appears that among the earliest organic remains there 

 existed an optical instrument of most curious construction, adapted 

 to the production of a peculiar kind of vision. We do not find 

 this instrument undergoing a series of experimental changes from 

 more simple into more complex forms : we find it created at the 

 very outset in the fulness of perfect adaptation to the uses and 

 condition of the class of creatures to which this kind of eye has 

 ever been and is still appropriate. (See Dr. Buckland's Bridge- 

 water Treatise, Geology and Mineralogy considered with refer- 

 ence to Natural Theology, vol. i. pp. 401-403.) 



contained in the cellular cavities or between the mem- 

 branous layers of the animal ; the size of the latter 

 being always proportionate to that of the shell. 



Nor were specimens of the class Radiata wanting 

 the Echinodermata being represented by the Hemicoa- 

 mites among other genera ; all of which, like existing 



llemlcosmites pyriformis. One-third natural size. 



types, were distinguished by their possession of a hard 

 coriaceous or leathery covering, a digestive and vascular 

 system, locomotive organs, and sexual distinctions. 



" In this group," says Mr. Patterson, " we find animals 

 of extremely dissimilar appearance associated together. 

 One species is attached, for a certain period, to a stem, 

 and resembles a polype, with its waving and sensitive 

 arms. In the common star-fish, or ' five fingers,' we 

 have the arms radiating from a common centre. In the 

 sea-urchins there are no arms, and the form of the 

 body is globular; and passing over some intermediate 

 gradations of figure, we reach creatures which in external 

 aspect resemble worms, and have even been classed as 

 such. At one extremity of the range the Echino- 

 dermata remind us of polypes creatures of inferior 

 organization ; at the other extremity they approach the 

 annulose animals, whose structure is of a higher grade." 



In these warm shallows of the Silurian ocean flourished 

 also several species of Pteropods (wing-footed), Hetero- 

 pods (paddle-footed), Graptolites, and Cystidians ; 

 all having sprung into life during the earlier times 

 of the Silurian period. These, however, possessed 

 no peculiar characteristics which call for a detailed 

 notice ; and we shall content ourselves with indicating, 

 in the following summary, the general groups and classes 

 of animals belonging to the lower (i.e., the earlier) 

 Silurian period, and the strata wherein their remains 

 are found interred. 



LACRENTIAN GROUP OF ROCKS. 



Gneiss, Hornblende, and Mica- 



schists, 



Foraminifera." 



CAMBRIAN GROUP. 



I.lanberis slates, and sandy strata 



Lingula flags, . 

 Tremadoc slates, 



Annelides (worms). 



1 Trilobites; Olenus; Cono- 



! coryphe paradoxides ; 



I Brachiopods ; Cystid- 



J eans. 



) Trilobites ; Bellerophon ; 



I Orthoceratites ; Theca. 



Foraminifera, a class of minute many-chambered shells, so 

 called from their perforated partitions. 



L: 



