MOLLUSCOIDA: DISTRIBUTION. 277 



According to Woodward : " The hingeless genera attained 

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

 vive (Lingula, Distina, 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 Terebratulidcz 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, Terebratulidce, attained its maximum 

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



