MOLLUSCOIDA ! DISTRIBUTION. 3 I 9 



the present day, and present us with an example of a group 

 which appears to be slowly dying out. Nearly two thousand 

 extinct species have been described, and the class appears to 

 have attained its maximum in the Silurian epoch, which is, 

 for this reason, sometimes called the " Age of Brachiopods." 

 Numerous genera and species are found also in both the 

 Devonian and Carboniferous formations. In the Secondary 

 Rocks Brachiopoda are still abundant, though less so than in 

 the Palaeozoic period. In the Tertiary epoch a still further 

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

 acquainted with a hundred living forms. Of the families of 

 Brachiopoda, the Productidce, Strophomenidce, and Spiriferidce 

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

 persistent is the genus Lingtila, which commences in the Cam- 

 brian 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, 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 Tere.bratulidce 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, Terebratnlidce. attained its maximum 

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



Of the families of the Brachiopoda, the Productidce and 

 Strophomenidce are exclusively Palaeozoic. The Spiriferida 

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

 finally disappear. The Lingulida commence in the Cambrian 

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

 nellidce, Craniadce, and Discinidcz commence in the Silurian 

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

 The Koninckidce are exclusively Triassic. The Thecididce ex- 

 tend from the Trias to the present day ; and the Terebratulida 

 appear to commence in the Devonian, and are well represented 

 by living forms. 



