ANCIENT LIFE ON THE EARTH 29 



The thecse also, which were at first straight and with 

 straight apertures, became curved and with curved 

 apertures, often produced into a spine, and in 

 the Silurian period the aperture was in some cases 

 still more complex. The species of Graptolites are 

 widely spread geographically, and occur in very 

 dissimilar rocks, such as limestones, shales, and 

 grits. Sometimes they are accompanied by a varied 

 fauna ; but in other places they occur in thin zones 

 without any other fossils ; while the different species 

 which characterise these zones are the same, and 

 have the same vertical distribution, wherever they 

 are found. The explanation of these facts appears 

 to be that the Graptolites floated on the surface, and 

 consequently were independent of the depth of the 

 sea and the nature of the sea bottom. We find ad- 

 ditional evidence in confirmation of this in the fact 

 that some of the early species were furnished with a 

 disc, which probably acted as a float. 



The Brachiopoda show a remarkable development 

 during both the periods under consideration. Orthis, 

 in which the triangular opening for the peduncle 

 remains open all through life, gave rise to the Rhyn- 

 chonellidce, which has a pair of deltidial plates in the 

 opening, and to the Strophomenidce , in which the 

 opening becomes, during growth, entirely closed by 

 a shelly plate, thus leaving the animal free. From 

 the Rhynchonellidce sprung, in the Silurian period, 

 the Terebratulidce, in which the deltidial plates 

 remain separate, and the Spiriferidce, in which they 

 unite during growth, and close the opening for the 

 peduncle, as in the Strophomenidce. 



