104 THE LIVING WORLD. 



In the Silurian (2), indeed, the Brachiopoda were more abundant than 

 any other group of animals, and the age is therefore sometimes called 

 the age of Brachiopoda. At this time they reached their culmination, 

 and have been declining ever since, though quite a number of them 

 are still in existence. It is especially interesting that some of the 

 genera existing to-day are not to be distinguished from those of the 

 very earliest rocks. (Lingula, Terebratula.) 



Bryozoa or Polyzoa. These form a group of small animals not 

 generally familiar except to students of natural history. They are 

 abundant to-day at the sea shore, and usually are mistaken for plants 

 by those unacquainted with them. They are small animals, but many 

 of them have a calcareous shell. They appeared in the Silurian (2), 

 and existed with no special change, though as the modern era 

 approached, the animals gradually assumed more the type of the 

 existing Bryozoa. Like the Brachiopoda, they form a fossil order 

 whose chief development occurred in the past, although they have 

 not yet become so widely extinct. 



Mollusca. 



Of all animals, we have the most complete geological record of the 

 mollusks. They were well developed as a group at the beginning of 

 the Silurian (2), and therefore their fossil record cannot tell us any- 

 thing of their early history, but of their history since the Silurian we 

 have a very full account. Their hard shells and aquatic habits have 

 adapted them especially well to preservation. We recognize five 

 classes of mollusks, whose history has been as follows : 



Lamellibranchia (oysters, clams, etc.). This group is characterized 

 by having two shells, and is common both in fresh and salt water. 

 The class was abundantly represented in the earliest Silurian (2), 

 many of our modern families being already in existence. Indeed, 

 quite a number of the genera then living still exist. By the close of 

 the Paleozoic (2-4) are found many others not so well known. With 

 the Mesozoic (5-8) there was a change. Many of the old forms dis- 

 appeared and new ones took their places. During the rest of the 

 geological ages there was an approach to the modern fauna. The old 

 forms did not entirely disappear, however, and some of the oldest fami- 

 lies continue to exist to-day. The Mesozoic is marked by the develop- 

 ment of the clams, quohogs, and other less well-known mollusks. The 

 modern forms have especially developed what is known as a siphon. 



