108 



THE AMERICAK MONTHLY 



[June, 



particles of food likely to be within 

 their reach. All species of this 

 group surround themselves with some 

 form of shell or test, and the fact to 

 which they owe their chief importance 

 is their ability to separate carbonate of 

 lime from its solution in the sea-water. 



Of their distribution it may be said 

 that with the exception of polar seas 

 it is as wide as that of the waters of 

 the ocean. The sounding-cup never 

 fails to bring them from the bottom, 

 and in some parts of the Atlantic 

 the mud dredged consists of as much 

 as 85 per cent, of foraminifera. 

 Wherever the ocean has rolled in 

 past geological ages since any living 

 thing has existed, they have been. 

 The chalk beds all over the world 

 are composed almost exclusivelv of 

 their remains. These chalk beds, in 

 this countiT, cover thousands of 

 square miles, and in some places are 

 9,000 feet in thickness. Thev are 

 probably not less extensive in other 

 parts of the globe. 



The nummulitic limestones extend- 

 ing in a vast bed on both sides of the 

 Mediterranean, through Northern 

 India and Central Asia, are principallv 

 shells of foraminifera and get their 

 name from a genus of large size, very 

 numerous and conspicuous through- 

 out the stratum. The Pyramids of 

 Egypt are built of this stone and rest 

 upon rock of the same structure, in 

 which the fossil foraminifera areeasilv 

 visible to the naked eve. It isprobaljle 

 that the subcarboniferous limestones 

 have the same origin. In short, the 

 weight of evidence is that the forami- 

 nifera have had more to do in forming 

 geological strata than all other animals 

 taken collectively. Moreover, if the 

 conclusions of Profs. Carpenter and 

 Dawson in regard to the Eozooii 

 Canadense are accepted, the forami- 

 nifera are the oldest in geological 

 time of known fossils. So these 

 minute shells, the product of the 

 simplest of animal organisms, are not 

 so insignificant in the economv of 

 nature as they might at first appear. 



Aside from their oreologfical im- 



portance and biological interest, they 

 attract attention by the beauty and 

 infinite variety of their forms, and 

 they illustrate better than any other 

 series of animals the endless varieties 

 that may be produced by the slight 

 but persistent modifications of the 

 mode of growth. 



In all attempts at classification of 

 such objects as these, external form 

 must necessarily be the governing 

 principle. There are, however, a few 

 prominent distinctions based on phys- 

 iological differences wdiich should be 

 considered. For instance, a large 

 group of these animals form their 

 tests of grains of sand, spicules of 

 sponge, or the shells of other forami- 

 nifera. They repeat in a rude way 

 nearlv all the forms taken by the more 

 delicate calcareous shells, but the 

 physiological distinction is of more 

 importance than the external resem- 

 blance and properly causes the testa- 

 ceous foraminifera to be classified 

 apart from the calcareous forami- 

 nifera. Another broad distinction is 

 based upon the arrangements for the 

 protrusion of the pseudopodia. In a 

 portion of the group the shell is ' im- 

 perforate,' by which is meant that there 

 is only one mouth-opening through 

 which the pseudopodia can be thrust 

 out. In the others the shell is por- 

 ous, or ' perforate,' studded all over 

 with minute openings, the largest not 

 more than j^^yVn o^ "^^ inch in diame- 

 ter, through which portions of the 

 bodv substance are extended for the 

 absorption of nutriment. These lat- 

 ter generally have a conspicuous 

 mouth-opening also, but this opening 

 is believed to serve simply as an exit 

 for the sarcodic substance in the pro- 

 cess of growth. These then consti- 

 tute the principal divisions based upon 

 physiological differences which can 

 be sustained, viz : into arenaceous and 

 calcareous, and into perforate and im- 

 perforate. Other distinctions are 

 based upon external form alone, and 

 it is interesting to consider by what 

 simple modifications the most aston- 

 ishinsT results are brousfht about. 



