FIRST FORMS OF LIFE. 33 



were any higher animals of that order (such as crabs, lobsters, 

 &c.,) yet in existence. 



Of the division Mollusca, the predominant form in point of 

 numbers was the bivalve order Brackiopoda, which is now 

 but slenderly represented upon earth. So numerous are the 

 specimens in the Silurian rocks, that an eminent geologist 

 calls this emphatically the Age of Brachiopods. ( n ) The ani- 

 mal is a humble one, having two shells, not connected by a 

 hinge, as is usual in superior bivalves, but kept together by a 

 bundle of fibres. Its destiny is to remain fixed at the bottom 

 of deep seas, and live upon nutritive particles, which it col- 

 lects by means of two spiral-shaped arms, extending from 

 the margin of its mouth, and from which the order has received 

 its name. The superior abundance of life in the depths of the 

 ocean, far from land, may be inferred, as a fact of this period, 

 from the comparative number of the brachiopod fossils. 



Of univalves, which, generally speaking, rank above the 

 bivalves, there are remains of all the three classes. The first 

 and humblest, Pteropoda, most of which are naked, and 

 therefore incapable of preservation in the fossil state, appear 

 only in a few slight conical shells, indicating an animal allied 

 to the genus Criseis, still common in the Mediterranean. Of 

 the next, Gasteropoda, there are many fossil species. There 

 are also representatives of the last class, Cephalopoda, amongst 

 which are now found some of the highest of the invertebrate 

 animals, as the nautilus, cuttle-fish, and poulp. The cepha- 

 lopods (orthoceratites, &c.,) pursuing a free-swimming life, 

 and highly organized for the catching and destroying of the 

 weaker marine animals, were the lords of the organic world 

 in their day. 



Such are the organisms of the Lower Silurian era, the first 

 age of organization upon earth of which any very distinct 

 memories have been left to us. There was as yet no fish nor 

 any other kind of vertebrated animal, nor any creature which 

 lived upon dry land. The zoology of the Upper Silurians is 

 only different in as far as it presents, for the most part, new 

 species of the same families, and a greater abundance of speci- 

 mens ; one rock (Wenlock limestone) is a mere mass of the 



D 



