LOWER PALAEOZOIC FAUNAS 143 



accelerated evolution of the short-lived Spiriferacea, 

 would be almost negligible. A few primitive Rhyn- 

 chonellids occur in the Ordovician, and the genera 

 Camarotoechia (PL vi. fig. i) and Wilsonia exhibited the 

 true Rhynchonella type in the Silurian. No members 

 of the Terebratulacea appeared until the Devonian. 

 Spiriferacea were, without doubt, the most elaborate of 

 Brachiopods. Ordovician types (with very few excep- 

 tions) agreed in having the brachidial spirals directed 

 inwards, as in Zygospira and Atrypa. Most families 

 originated in the Silurian. The true "Spirifers" are 

 there represented by D el thy r is, Eospirifer and Cyrtia, 

 the relatively long-lived Suessiidae by Cyrtina, and the 

 elaborate Meristellidae by all the acmaic complexity of 

 such genera as Nucleospira and Meristina. Atrypa 

 (PL x. fig. i), surviving from the Ordovician, rivals 

 Conchidium as an Upper Silurian rock-former. 



(H) MOLLUSCA 



No other phylum can show so voluminous a record as 

 the Mollusca. Although subordinate to Brachiopoda in 

 Palaeozoic times, the three classes Pelecypoda, Gastro- 

 poda and Cephalopoda reversed the proportions in an 

 overwhelming manner in the Mesozoic, and certain 

 groups of the two first series are only less abundant 

 than Insects at the present day. A few types live on 

 land, many in fresh water, and a multitude in the sea. 

 Some marine forms are littoral, others pelagic ; few are 

 abysmal. Certain families flourish in the muddy water 

 of estuaries, others prefer rocks or reefs. The majority 

 of Molluscs secrete relatively massive shells, and, in 

 spite of the preference for Aragonite shown by some 

 groups, are better adapted for fossilization, by structure 

 and habitat, than most other Invertebrates. In spite of 



