128 



HISTORICAL PALAEONTOLOGY. 



come upon the scene for the first time, or which have now a 

 special development, are Spirifera and Pentamerus. In the 



Aa, 



Fig. 66. Upper Silurian Polyzoa. 1, Fan-shaped frond of Ehinopora verrucosa; la, 

 Portion of the surface of the same, enlarged ; 2 and 2a, Phcenopora ensifonnis, of the 

 natural size and enlarged ; 3 and 3a, 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.) 



first of these (fig. 69, b, c), one of the valves of the shell (the 

 dorsal) is furnished in its interior with a pair of great calca- 

 reous spires, which served for the support of the long and 



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

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



fringed fleshy processes or " arms " which were attached to the 

 sides of the mouth. * In the genus Pentamerus (fig. 70) the 

 shell is curiously subdivided in its interior by calcareous 

 plates. The Pentameri commenced their existence at the very 

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



* In all the Lamp-shells the mouth is provided with two long fleshy 

 organs, which carry delicate filaments on their sides, and which are 

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

 and it is from their presence that the name of " BracJiiopoda " 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 Sbirifers, and in other ancient 

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

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

 serve as one of the special characters distinguishing the Brachiopods from 

 the true Bivalves (Lamcllibranchiata). 



