126 HISTORICAL PALAONTOLOGY. 
shell is curiously subdivided in its interior by calcareous 
plates. The /entameri commenced their existence at the very 
Fig. 66.—Upper Silurian Polyzoa. 1, Fan-shaped frond of Rhixofora verrucosa; 1a, 
Portion of the surface of the same, enlarged; 2 and 2a, Phenopora enstformts, of the * 
natural size and enlarged; 3 and 3a, Helofora fragilis, of the natural size and en- 
larged ; 4 and 4a, Ptilodictya rarifora, 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. drachion, 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 Sfirifers, and in other ancient 
genera, this apparatus is coiled up into a complicated spiral (fig. 67). It 
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. 
is these ‘‘arms,” with or without the supporting loops or spires, which 
serve as one of the special characters distinguishing the Lrachiopods from 
the true Bivalves (Lamellibranchiata). 
