RELATION OF BIOLOGY TO GEOLOGICAL INVESTIGATION. 355 



As a rule the Penseidse inhabit marine waters, but sonic of the spe- 

 cies are known to range into fluviatile waters. 



The Stomatopoda have considerable range of habitat. Some of the 

 Mysidae range from marine into brackish waters, and at least one spe- 

 cies is found in inland fresh waters. 



The Squillidae are, as a rule, marine forms, but some of them range 

 into brackish waters. 



The greater part of the Isopoda inhabit marine waters, but many 

 genera are confined to fresh waters, while all the members of one family, 

 the Oniscidae, are of terrestrial habitat. A considerable number of 

 genera also range into brackish water. 



All known members of the Anisopoda are denizens of marine waters. 



A considerable proportion of the species and many genera of the 

 Amphipoda are denizens of fresh water while most of the others are 

 confined to a marine habitat, many ranging into brackish waters. 



Tbe subclass Entomostraca embraces a greater proportion of denizens 

 of nonmarine waters than does the subclass Malacostraca. They are 

 also of special interest to the geologist because representatives of at 

 least one suborder are found in much earlier formations than are any 

 members of the other subclass. 



The Girripedia are so generally regarded by geologists as being ex- 

 clusively denizens of marine waters, or of those which are of nearly 

 full marine saltness, that they rarely hesitate to accept the presence of 

 fossil remains of any crustacean referable to this order in any forma- 

 tion as evidence of its marine origin. One species of Balanus, how- 

 ever, has been found in comparative abundance ranging from marine 

 waters far up the St. John's river in Florida, and thriving there in 

 fresh water. This case is apparently as exceptional as that, for exam- 

 ple, of the Teredinoid and Arcoid mollusks in the lower Ganges and of 

 Selachian fishes in Lake Nicaragua, but it may be that other similar 

 cases will be found among the Girripedia. Still, in the absence of con- 

 flicting evidence geologists are probably justified in regarding Cirriped 

 remains as indicating marine conditions. 



The Copepoda are mostly minute animals, and inhabit both fresh and 

 salt waters. They are usually provided with a ehitiuous or membra- 

 nous covering which, with few exceptions, is too delicate to be well 

 preserved in sediments after the death of the animal. It is probable, 

 therefore, that few fossil remains of these animals have ever become 

 fossilized, and these are likely to escape discovery because of their 

 minuteness and delicacy. 



Various forms of the bivalve Crustacea constituting the Ostracoda 

 are found in both fresh and marine waters, and also in the brackish 

 water of estuaries. They are often gregarious, immense numbers being 

 often found together. Their former abundance was also great, as is 

 shown by the multitudes of fossil forms that are sometimes found in 

 the sedimentary rocks of various ages. 



