NATURAL HISTORY OF CREATION. 53 



vestiges of fuci, or sea-plants, and of fishes. This group 

 of rocks has been called by English geologists, the 

 Silurian St/stem, because largely developed at the surface 

 of a district of western England, formerly occupied by a 

 people whom the Roman historians call Silures. It is 

 a series of sandstones, limestones, and beds of shale 

 (hardened mud), which are classed in the following sub- 

 groups, beginning with the undermost : — i, Llandeilo 

 rocks (darkish calcareous flagstones) ; 2 and 3, two 

 groups called Caradoc rocks ; 4, Wenlock shale ; 5, 

 Wenlock limestone ; 6, Lower Ludlow rocks (shales and 

 limestones); 7, Aymestry limestone; 8, Upper Ludlow 

 rocks (shales and limestone, chiefly micaceous). From 

 the lowest beds upwards, there are polypiaria, though 

 most prevalent in the Wenlock limestone ; brachiopo- 

 dous mollusks, a vast number of genera (including 

 terebratula, pentamerus, spirifer, orthis, leptsena) ; 

 cephalopoda, of several orders and many genera (includ- 

 ing turritella, orthoceras, nautilus, bellerophon) ; Crus- 

 tacea, all of them trilobites (including trinucleus, 

 asaphus, calymene). The cephalopoda are the most 

 highly organised of the mollusca, possessing in some 

 families an internal osseous skeleton, together with a 

 heart, and a head having some resemblance in form and 

 armature to that of the parrot tribes. This order was 

 carnivorous, and acted the part of a police in keeping 

 down the redundant life of the early seas. The trilobites 

 have attracted much attention in consequence of the gi-eat 

 variety of species, and the peculiar structure of the eye, 

 Avhich unequivocally tells that the sea was a clear medium 

 at that time as at the present, and that light was then, 

 in its general nature and relation to the vision of animals, 

 the same as it now is. A little above the Llandeilo rocks, 

 there have been discovered certain convoluted forms, 



