THE SHELL STRUCTUEE .41 



example, in Uliizaiiiviina. The material thus used 

 in building the test may be sand grains (volcanic 

 dust, quartz, calcareous, or glauconite grains), sponge 

 spicules, or even the small tests of other Foramini- 

 fera, as in the case of Hliizanvmina and Heoi^liax. 

 In the genus Carteiiiia the organism, however, does 

 not rely on its surroundings for building material, 

 but itself secretes innumerable delicate little fusiform 

 spicules of a calcareous nature, with which to form 

 its test, 



A noticeable feature of all arenaceous Fora- 

 minifera is the invariably smooth internal lining 

 to the test, which is glazed over with a film of 

 chitinous or sarcodic material ; the exterior, how- 

 ever, may vary from the neatly cemented and 

 smooth-surfaced test of Trochajiimina to the coarsely 

 agglutinated covering of a Haplojjliragmiuni or a 

 Heophax. 



Many arenaceous species are isomorphous, as 

 regards external form, with species of other and 

 distinct groups, such as HaplophragmiuoR with 

 Anomalina, or Placop)silina with Nuhecularia. 



That the arenacea is not a true zoological group 

 is borne out by the fact that both arenaceous and 

 hyaline types of shell structure exist in the same 

 natural group or genus, as seen in Textnlaria or 

 Verneiiilina. 



The whole of the families of the Astboehizid.e, and 

 the LiTuoLiD.E, and also a part of the Textulaehd.e, 

 have arenaceous tests. 



4. Chitinous Forms. — The members of the family 



