1 84 HISTORICAL PALAEONTOLOGY. 



netted fronds oi Fenestella, Retepora, and Polypora (fig, 126, a) 

 are highly characteristic, as are the slender toothed branches 

 of Glauconome (fig. 126, b). A more singular form, however, is 

 the curious Archimedes (fig. 126, c), which is so characteristic 

 of the Carboniferous formation of North America. In this re- 

 markable type, the colony consists of a succession of funnel- 

 shaped fronds, essentially similar to Fenestella in their structure, 

 springing in a continuous spiral from a strong screw-like vertical 

 axis. The outside of the fronds is simply striated ; but the 

 branches exhibit on the interior the mouths of the little cells' 

 in which the semi-independent beings composing the colony 

 originally lived. 



The Brachiopods are extremely abundant, and for the most 

 part belong to types which are exclusively or principally 

 Paleozoic in their range. The old genera Strophomena, Ortkis 

 (fig. 127, ^), At/iyris (fig. 127, e), Rhynchonella (fig. 127, g), and 

 Spirifera (fig. 127, h)^ are still well represented — the latter, in 

 particular, existing under numerous specific forms, conspicuous 

 by their abundance and sometimes by their size. Along with 

 these ancient groups, we have representatives — for the first time 

 in any plenty — of the great genus Terebratula (fig. 127, d), 

 which underwent a great expansion during later periods, and 

 still exists at the present day. The most characteristic Car- 

 boniferous Brachiopods, however, belong to the family of the 

 Productida^ of which the principal genus is Prodiida itself. 

 This family commenced its existence in the Upper Silurian 

 with the genus Choiietes, distinguished by its spinose hinge- 

 margin. This genus lived through the Devonian, and flourished 

 in the Carboniferous (fig. 127,/). The genus Prodiicta itself, 

 represented in the Devonian by the nearly allied Productella, 

 appeared first in the Carboniferous, at any rate in force, and 

 survived into the Permian ; but no member of this extensive 

 family has yet been shown to have over-lived the Palaeozoic 

 period. The Productce of the Carboniferous are not only ex- 

 ceedingly abundant, but they have in many instances a most 

 extensive geographical range, and some species attain what 

 may fairly be considered gigantic dimensions. The shell (fig. 

 127, ^ and b) is generally more or less semicircular, with a 

 straight hinge-margin, and having its lateral angles produced 

 into larger or smaller ears (hence its generic name — " cochlea 

 prodiicta'"). One valve (the ventral) is usually strongly convex, 

 whilst the other (the dorsal) is flat or concave, the surface of 

 both being adorned with radiating ribs, and with hollow 

 tubular spines, often of great length. The valves are not 

 locked together by teeth, and there is no sigri in the fully- 



