LOWER PALAEOZOIC FAUNAS 141 



Palaeozoic; during that era they rivalled, in numbers 

 and differentiation, the later achievements of Mollusca. 



The simple, inarticulate, largely corneous order 

 Atremata enjoyed in the Lower Palaeozoic such 

 repressed prominence as it has attained. After the 

 close of the Silurian period, but one family lingered to 

 perpetuate the stock in the existing fauna. The 

 exceedingly rapid, and relatively profuse, specialization 

 of families in Cambrian and Ordovician times is 

 suggestive of neanic acceleration ; and, since the 

 Atremata are undoubtedly ancestral to all other orders 

 of Brachiopods, indicates a late pre-Cambrian stage 

 as the time of inception of the phylum. The Lower 

 Cambrian Rustella (unknown from Britain) cannot be 

 far removed in morphology from the first definite 

 Brachiopod. Its contemporary Kutorgina shows modi- 

 fications pointing towards the Protremata. The most 

 abundant family of Cambrian Brachiopods, the Obolidae, 

 is represented by Lingulella (PL ix. fig. 8) in the middle 

 and upper Cambrian of this country, and was continued 

 into the Silurian by the massive, phylogerontic stock of 

 the Trimerellids. Some early Obolids seem to have 

 given rise to the Neotrematous group Obolellidae. It 

 is probable, though less certain, that the same series of 

 Atremata are on the direct line of descent of the 

 Telotremata. The Lingulacea, destined to survive with 

 scarcely any important modification until the present 

 day, seem to be continuous with the Obolid group 

 represented by Lingulella, appearing first, and in 

 greatest profusion, in the Ordovician. Lingula itself 

 was often exceedingly abundant in shallow Silurian seas, 

 and attained much specific differentiation during that 

 period. In particular, the beds of Lingula cornea and L. 

 minima in the Uppermost Silurian are deserving of notice. 



The small, curiously specialized members of the 



