250 MANUAL OF ZOOLOGY. 



diminution takes place, and at the present day we are not 

 acquainted with a hundred living forms. Of the families of 

 Brachiopoda, the Produdidce, Strophomenidce and Spiriferidce 

 are the more important extinct types. Of the genera, the 

 most persistent is the genus Lingula, which commences in the 

 Cambrian Rocks, and has maintained its place up to the pre- 

 sent day, though it appears to be gradually dying out. 



According to Woodward : ' the hingeless genera attained 

 their maximum in the Palaeozoic age, and only three now sur- 

 vive (Lingula, Discina, Crania) the representatives of as 

 many distinct families. Of the genera with articulated valves, 

 those provided with spiral arms appeared first, and attained 

 their maximum while the Terebratulid.ee were still few in num- 

 ber. The subdivision with calcareous spires disappeared with 

 the Liassic period, whereas the gfnus Rhynchonella still exists. 

 Lastly, the typical group, Terebratulidce, attained its maximum 

 in the Chalk period and is scarcely yet on the decline.' 



