1 2 PALEONTOLOGY. 



enough to be washed, down to the Lower Silurian; and in the 

 hard limestones and marbles they can be detected in polished 

 sections, and in thin slices laid on glass. The greater part of 

 these shells are microscopic, but some of the large extinct 

 foraminifers, called, from resembling a piece of money, * Num- 

 mulites," are two inches in diameter. 



The generic divisions in common use for these shells are 

 founded upon the plan of growth, or mode of numerical 

 increase of the chambers. The following are the primary 

 groups of Rhizopoda in the system of d'Orbigny : — 



1. Monostega. — Body consisting of a single segment: shell of 



one chamber. 



2. Stichostega. — Body composed of segments disposed in a 



single line : shell consisting of a linear series of 

 chambers. 



3. Helicostega. — Body consisting of a spiral series of segments : 



shell made up of a number of convolutions. 



4. Entomostega. — Body consisting of alternate segments spirally 



arranged : shell chambers disposed on two alternating 

 axes forming a spiral. 



5. Enallostega. — Body composed of alternate segments not 



forming a spiral : chambers arranged on two or three 

 axes which do not form a spiral. 



6. A gathistega. — Body consisting of segments wound round 



an axis : chambers arranged in a similar manner, each 

 investing half the entire circumference. 



A somewhat different arrangement has been adopted by 

 Schultze, who divides the Polythalamia into three sections, 

 viz. — ■ 



L. Helicoidea, including those forms in which the several 

 chambers of the shell are arranged in a convolute 

 series, and answering to the last four orders of d'Or- 

 bigny. 



