BBCC ii. AREXACEOUS FORAMINIFERA. 37 



The spiral tubes of this genus take various forms, and 

 in some cases are divided into chambers. 



The members of the genus Lituola exude from their 

 surfaces a thick coat of cement with a quantity of 

 siliceous particles roughly imbedded in it, but in some 

 instances the particles are so uniform in size and shape, 

 and are so methodically arranged, that the surface re- 

 sembles a tesselated pavement. The usual form of the 

 Lituola is a mere string of oval convex chambers in- 

 creasing gradually in size, and fixed to shells and corals 

 by their flat surfaces. In some instances the shells, or 

 rather the substitutes for shells, take a nautiloid form, 

 and become detached from the foreign bodies to which 

 they were attached. In the highest forms of this genus 

 the chambers are divided by secondary partitions. 



The typical form of the genus Yalvulina is a three- 

 whorled, three-sided pyramidal shell, with three cham- 

 bers in every turn of the spire. The aperture is large 

 and round, with a valve of smaller size attached by 

 a tooth of shell to its rim. The creature itself has an 

 exceedingly thin perforated vitreous shell, covered by 

 an incrustation of calcareous particles, which so entirely 

 blocks up the perforations that it can only extend its 

 pseudopodia through the mouth of its shell. 



Order of Vitreous Foraminifera. 



Nearly all the Foraminifera on the British coasts 

 belong to the Vitreous or Perforated order, which con- 

 sists of three natural families and many genera. Their 

 shells are vitreous, hyaline, and generally colourless, 

 even although the substance of the animal is deeply 

 coloured ; in some species both the animal and its shell 

 are of a rich crimson. The glassy transparency of the 

 shells would be perfect were they not perforated by 

 numerous tubes running from the interior of the cham- 



