126 



HISTORICAL PALAEONTOLOGY. 



shell is curiously subdivided in its interior by calcareous 

 plates. The Pentameri commenced their existence at the very 



Fig. 66. Upper Silurian Polyzoa. i, Fan-shaped frond of Rhinopora -uemicosa; ia, 

 Portion of the surface of the same, enlarged ; 2 and za, Phcenopora ensij'ormis, of the 

 natural size and enlarged ; 3 and 3,7, Helopora fragilis, of the natural size and en- 

 larged ; 4 and 4^, Ptilodictya raripora, of the natural size and enlarged. The speci- 

 mens are all from the Clinton Formation (May Hill Group) of Canada. (Original.) 



close of the Lower Silurian (Llandovery), and survived to the 

 close of the Upper Silurian ; but they are specially character- 

 istic of the May Hill and Wenlock groups, both in Britain 

 and in other regions. One species, Pentamerus galeatus, is 

 common to Sweden, Britain, and America. Amongst the 

 remaining Upper Silurian Brachiopods are the extraordinary 



usually coiled into a spiral. These organs are known as the "arms," 

 and it is from their presence that the name of " Brachiopoda " is derived 

 (Gr. brachion, arm ; podes, feet). In some cases the arms are merely coiled 

 away within the shell, without any support ; but in other cases they are 

 carried upon a more or less elaborate shelly loop, often spoken of as the 

 "carriage-spring apparatus." In the Spirifors, and in other ancient 

 genera, this apparatus is coiled up into a complicated spiral (fig. 67). It 



Fig. 67. Spirifera-fiysfericri. The right-hand figure shows the interior of the 

 dorsal valve, with the calcareous spires for the support of the arms. 



is these "arms," with or without the supporting loops or spires, which 

 serve as one of the special characters distinguishing the J3rachiopods from 

 the true Bivalves {Lamellibranchiata). 



