1 66 INVERTEBRATE PALAEONTOLOGY 



out into three families, two of which (and an important 

 part of the third) are restricted to the Upper Palaeozoic. 

 Centronella, Stringocephalus, Megalanteris and Dielasma 

 (PI. xi. fig. 8) represent these early, short-lived experi- 

 ments of the series. True " Terebratulas " are unknown 

 before the Trias. Among the Spiriferacea, the best 

 known and most fully specialized representatives are of 

 Devonian and Carboniferous age. The Atrypidae 

 lingered into the Devonian, but were overshadowed by 

 the Spiriferidae. Spirifer (PI. xi. fig. 9) itself (in the 

 broad sense of the term) is scarcely less common in the 

 Carboniferous Limestone than Productus, and attained 

 peculiar proportions in such sections as Syringothyris. 

 The Suessiidae, which survived into the Jurassic, are 

 rarely abundant, and the Meristellidae dwindled to 

 extinction in the Devonian. Two new families arose 

 in that period. The anomalous Uncitidae, with dorso- 

 lateral pouches and rostrate form, are typically Devonian, 

 but persisted to the Permian; while the superficially 

 simple but inwardly complex Athyridae are among 

 the most characteristic Upper Palaeozoic Brachiopods. 

 Athyris is locally abundant in the Carboniferous Lime- 

 stone ; its aberrant, often cemented, descendants reached 

 their phase of gerontic acceleration in the Trias. 



(H) MOLLUSCA 



Upper Palaeozoic Mollusca show a marked increase 

 in numbers and variety upon the exiguous fauna of the 

 preceding era. Opportunities for discovery of terrestrial 

 and fresh-water types have but slight influence on this 

 quality. Many genera and families of Pelecypods that 

 arose in the Silurian reached their acmes in the Car- 

 boniferous, while an even greater number of fresh 

 types appeared. Gastropods maintained their previous 

 qualities, but showed small tendencies to differentiation 



