86 INVERTEBRATE PALAEONTOLOGY 



the only Brachiopods arc Lingulac\ Orthoceras disappears, 

 and the bulk of the fauna is composed of Pelecypods, 

 with occasional Tubicolous Annelids and Eurypterid 

 Arthropods. The " Ludlow Bone Bed " gives striking 

 testimony to almost catastrophic changes at this stage, 

 and in the true Downtonian that supervenes, even 

 Pelecypods have gone; Lingula alone survives, repre- 

 sented by dwarfed forms such as L. minima. The 

 micaceous strata of the Passage Beds enclose L. cornea, 

 and then almost all palaeontological evidence is lost in 

 the marls of Lake Monmouth. The gradual transition 

 from marine, through brackish, to fresh-water conditions 

 is very clearly shown in this faunal sequence. The 

 higher Brachiopods are exclusively marine, and dis- 

 appear at the first suspicion of " estuarine " contamina- 

 tion. Many Pelecypods (especially the Mytilidae, a 

 group nearly related to Modiolopsis] are indifferent to 

 dilution of sea-water, although they cannot survive when 

 sweetness has been quite attained. The Cephalopods 

 are as definitely marine as the Brachiopods; but 

 Lingula, one of the simplest members of the latter 

 phylum, may well have possessed the catholicity of 

 taste common to most lowly forms. A somewhat 

 similar sequence, carried out with other genera and in 

 the reverse direction, can be traced through the Rhaetic 

 faunas from the Tea Green Marls to the Lower Lias. 



(Ill) MARINE FAUNAS 



The life of the sea can be grouped broadly Into 

 two zones, each capable of further subdivision. The 

 two main provinces correspond fairly closely with the 

 detrital and pelagic zones of sedimentation respectively. 

 Forms inhabiting the littoral belt can be separated into 

 those demanding a " clean " environment, and those 



