LOWER PALAEOZOIC FAUNAS 147 



zoic forms, although such types as Patella and Trochus 

 are common at the present day. One of the simplest of 

 the group was Bellerophon, a coiled shell resembling that 

 of a Nautiloid in shape. It was not uncommon in the 

 Ordovician and Silurian, but became more important in 

 the Upper Palaeozoic. The long-lived genus Pleuro- 

 tomaria appeared first in the Silurian, but its acme was 

 in the Mesozoic. Murchisonia and Euomphalus are two 

 genera that originated in the Lower Palaeozoic (the former 

 perhaps in the Cambrian), but were better developed in 

 the Carboniferous. Platyschisma and Omphalotrochus 

 (PI. x. fig. 3) both survived to the last-named period ; they 

 were abundant in the Middle and Upper Silurian. 



The Ctenobranchiate section is divided into five super- 

 families on radular characters. Three of these groups 

 seem to have been present in the Lower Palaeozoic, but 

 it is impossible to be certain of the systematic position 

 of many genera. Gymnoglossa, which are apparently 

 degenerate in structure and habits, may have been repre- 

 sented by Macrocheilus and Loxonema in the Silurian, 

 but these genera were both more abundant in the 

 Carboniferous. The multidentate Ptenoglossa, a small 

 group of usually rare forms, seem to include Holopella^ 

 which is fairly common in the Silurian, particularly 

 in the Kirkby Moor Flags of Westmorland. The 

 Taenioglossa, the largest series of later Gastropods, 

 showed possible members of the Littorinidae in the 

 Ordovician, and somewhat problematical Capulidae in 

 the Cambrian (Capulus and Stenothecd) and Silurian 

 (Platyceras}. These three genera were mainly Palaeo- 

 zoic, but Capulus is still extant. 



Cephalopoda were far from rare in the Lower 

 Palaeozoic, but known forms seem restricted to the 

 Nautiloid branch of Tetrabranchiata. The living genus 

 Nautilus is the sole survivor of an extensive series of 



