UPPER PALAEOZOIC FAUNAS 171 



tion in importance. Both disappeared completely in 

 the Permian. The failure of these types was in large 

 measure counterbalanced by the rise of Xiphosura and 

 Scorpionida among Arachnids, and by the appearance 

 of Myriopods and Insects. The swamp-conditions of 

 Coal Measures gave unusual opportunities for preserva- 

 tion of the last two series ; their absence from Lower 

 Palaeozoic horizons may well be due to lack of evidence 

 rather than to actual non-existence. 



Hypoparian Trilobites were represented in the 

 Devonian by the solitary genus Harpes, a form showing 

 many affinities with the Trinucleidae of the Ordovician. 

 Harpes comprises in a curious way features of over- 

 specialization in the cephalon with reversionary (or 

 retarded) tendencies in the pygidium. Opisthoparia 

 maintained four families in the Devonian ; these were 

 reduced to three in the Carboniferous, and one (Proetidae) 

 in the Permian. Goldius (usually called Brontes), the 

 genus that failed in the Devonian, shows clearly the 

 gerontic quality of a large pygidium. The extra- 

 ordinarily spinous Lichadidae and Odontopleuridae, 

 which disappeared in the Lower Carboniferous, show 

 comparable over-specialization in a different direction ; 

 while Carboniferous and Permian Proetidae combine the 

 features of multiple pygidia with warty outgrowths from 

 the carapace. Phillipsia (PL viii. fig. 4, and PI. xii. 

 fig. 7) and Griffithides are two well-known genera from 

 the Carboniferous Limestone. Proparia disappeared in 

 the Devonian, but were represented by three of their 

 four families in that period. Spinous species of Homa- 

 lonotus marked the decline of the Calymenidae, Crotalo- 

 cephala is the end-form of the Cheiruridae ; while a fairly 

 extensive series of Phacopidae showed gerontic features 

 in simplification of the glabella, warty ornamentation, 

 and, in some cases, degeneration of the compound eyes. 



