34 INVERTEBRATE PALAEONTOLOGY 



is possible to discriminate between fossil-bands due to 

 the mode of life (or manner of death) of the organisms 

 concerned, and those produced by physiographical agents 

 working on discarded shells. As a rule, the former 

 types consist of well-preserved specimens of a single 

 species, while the others include a variety of more or 

 less damaged fossils. 



The local and congested habits of life of many 

 sedentary Invertebrates are illustrated by the familiar 

 mussel and oyster beds of British shores. Dredging in 

 deeper water proves that large areas of the sea-floor 

 below low-tide mark may be covered with Pecten or 

 Echinus to the virtual exclusion of other types. More- 

 over, it is by no means uncommon to find that the 

 dredge brings up nothing, or dead shells only, from a 

 place that yielded a rich harvest of living specimens at 

 a previous haul. Those Invertebrates that lead more or 

 less sedentary lives when adult, but have free-swimming 

 larval stages, are apt to congregate into clusters whose 

 situation changes periodically, perhaps with each genera- 

 tion. The evidence available indicates that Echinoderms, 

 Brachiopods and Pelecypods often show these habits, 

 in coastal or deep water. In a region where rock- 

 deposition is in progress, it is easy to account in this 

 way for inclusion of layers composed of one or two 

 species of shells in sediment which, above and below, 

 contains few of their remains. 



Again, if some rapid or cataclysmic change occurs in 

 the surroundings of a faunal assemblage, sudden death 

 will overtake all those incapable of speedy escape. Such 

 disasters are not likely to come frequently, but they may 

 occasionally overwhelm inhabitants of coastal regions. 

 Sudden outpouring of mud from a river in flood might 

 smother organisms unable to swim, and rapid shifting 

 of sand-shoals in a storm would have a like effect. 



