

MOLLUSCA: BRACHIOPODA. 395 



than a hundred living forms. Of the families of Brachiopoda, 

 the ProductidcCy 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 present 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, Distinct, 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 Terebratnlida were still few in number. 

 The subdivision with calcareous spires disappeared with the 

 Liassic period, whereas the genus Rhynchonella still exists. 

 Lastly, the typical group, Terebratnlid<z, attained its maximum 

 in the Chalk period, and is scarcely yet on the decline." 



Of the families of the Brachiopoda, the Productidcz and 

 Strophomenida are exclusively Palaeozoic. The Spiriferidcz 

 are mainly Palaeozoic, but extend into the Lias, where they 

 finally disappear. The Lingulidce commence in the Cambrian 

 period, and have survived to the present day. The Rhyncho- 

 nellidcE, Craniadce, and Disrinida commence in the Silurian 

 period, and are represented by living forms in existing seas. 

 The Thetidiidce extend from the Trias to the present day ; and 

 the Terebratulida appear to commence in the Upper Silurian, 

 and are well represented by living forms. 



