356 REPORT OF NATIONAL MUSEUM, 1892. 



Most of the Phyllopoda inhabit fresh waters, but some live in marine, 

 and some in brackish waters. They are mostly small or minute Crus- 

 taceans but being gregarious they are often found together in great 

 numbers. Their paleontologicai value is similar to that of the Ostra- 

 coda except that the latter are more prevalent in the paleozoic forma- 

 tions. 



Nearly or quite all the Crustacea of the earlier geological ag-3S are 

 referable to the subclass Gigantostraca, but its living representatives 

 are only the Xiphosura, and these are confined to two species of the 

 genus Limnlus. Although the Malacostraca are more conspicuously 

 abundant than all other living Crustaceans, fossil remains referable to 

 any divisions of this subclass are comparatively rare in strata of any 

 age and none have been found in rocks of earlier than Carboniferous 

 age. On the contrary, the Gigantostraca, which are represented by 

 only two known living species, existed in great abundance in the Cam- 

 brian, Silurian and Devonian ages, when they were represented by 

 numerous genera and families, especially of the trilobites. 



It is therefore evident that a knowledge of the different kinds of 

 aquatic habitat of living crustaceans affords little direct information 

 concerning that of those which lived daring the three earlier geological 

 ages just mentioned. Consequently all the obtainable evidence of this 

 kind is derived from the remains of their fauna! associates. Much the 

 greater proportion of all those ancient crustaceans, including all of the 

 trilobites, are thus assumed to have lived in marine waters, but the 

 usual absence of immediately associated forms that can be with cer- 

 tainty assigned to either a marine or nonmarine origin has left in 

 doubt the question as to the character of the water in which others of 

 the Gigantostraca lived. 



Annelida. — The members of this class which possess such skeletal or 

 protective parts as are most likely to resist decomposition are the Tubi- 

 cola, all of which secrete a shell, usually calcareous, much resembling 

 the shells of gasteropod mollusks. They are all denizens of saline 

 waters, mostly those of the open ocean. By means of the partly chiti- 

 nous covering of certain of the Errautia or roving worms, their bodily 

 form may occasionally be preserved after the death of the animal in the 

 sediments which formed its habitat while living; and their presence in 

 such sediments is often indicated by their burrows when the animals 

 themselves are not discovered. 



So generally are the Annelida denizens of marine waters that the 

 presence of the remains or burrows of any of them in a geological for- 

 mation is regarded by geologists as indicating its marine orig u. 



CCELENTERATA. 



The Hydrozoa only of the somewhat numerous orders embraced in 

 this subkingdom are represented in fresh waters. These fresh water 

 representatives are all minute, and are not furnished with skeletal parts 



