22 PROTOZOA. FORAMINIFERA 



seen (fig. 3, A, C) ; or it may be that more chambers are shown on 

 one side than on the other. The external features of the shell 

 consequently vary considerably, and on this account the forms in- 

 cluded under the term Miliola are now regarded as constituting a 

 number of distinct genera to which the following names have been 

 given : — Biloculina, Fabularia, Spi?'oloculina, Ifiliolina, Quinquelocu- 

 lina, etc. Trias to present day. Ex. Miliolina seminulum, Eocene 

 to present day ; Biloculina ringens, Eocene to present day ; Spiro- 

 loculina planulata, London Clay to present day. 



Orbitolites. Shell discoidal, generally rather large, composed 

 of either a small spiral part at the centre, or of one or more large 

 central chambers, around which are many concentric rings divided into 

 numerous chambers ; the chambers of adjacent rings communicate 

 by radial openings, and at the external margin of the last ring are 

 pores opening to the exterior. Above and below this layer of 

 chambers there may be another layer of smaller chambers, arranged 

 concentrically. Upper Cretaceous to present day. Ex. 0. compla- 

 nata, Eocene. 



Alveolina. Shell fusiform or elliptical, sometimes nearly 

 globular, composed of many whorls coiled around the long axis of 

 the shell ; each whorl completely covers the one preceding it, and is 

 divided into long chambers by partitions parallel with the axis of 

 the shell ; these are divided into smaller chambers by partitions at 

 right angles to the others. Chalk to present day ; chiefly Eocene. 

 Ex. A. boscL Eocene. 



II. Arenaceous Forms. 



Shell composed of grains of sand or other particles 

 cemented together by chitinous, calcareous, or ferruginous 

 material. 



Saccammina. Shell usually free, compact, formed of a single 

 spherical, pyriform, or fusiform chamber with a projecting aperture, 

 or of a number of chambers united end to end. Surface smooth or 

 nearly smooth. Ordovician, Carboniferous, and living. Ex. S. 

 fusuliniformis (= carter?'), Carboniferous Limestone. 



