8 NORTH AMERICAN INDEX FOSSILS. 



Subclass i. Foraminifera d'Orbigny. 



The shell may consist of a single chamber (unilocular) or of 

 many chambers (multilocular). The chambers may be arranged in 

 a straight line (nodosarian type, Fig. i), wound in a horizontal 

 manner (nautiloid type, Figs. 2, 3), in a spiral manner (trochoid 



Fk;s. 1-6. Diagrams illustrating types of Foraminiferal Shell (after Bagg). 



type, Fig. 4), alternating in a double series (textularian type, Fig. 

 5), regularly embracing the preceding chamber (milioloid, Fig. 6), 

 irregularly clustered (globigerinoid, Fig. 14), simple globular (orbu- 

 linoid, Fig. 13), or wrapped about an imaginary axis — either disk- 

 like (orbitoid, Fig. 19) or spindle-shaped (fusoid, Fig. 18). The 

 shell is either pierced by numerous pores or foramina (perforate) or 

 is imperforate. A terminal or oral opening is generally present. 

 The shell is a secretion of lime or results from cementation of sand 

 grains (Fig. 1 1). 



Owing to the fact that the Foraminifera vary but slightly from the 

 upper Paleozoic to the present, they are of very little use as index 

 fossils. A few of the more abundant genera are here given in 

 order that they may be recognized as such. A few species which 

 are restricted in their geological range and are found in great 

 abundance are described. 



Literature. 



1876. Brady, W. B. Monograph of Carboniferous and Permian 



Foraminifera, Pdlaeontographical Society Monographs. 

 1884. Brady, W. B. Report on Foraminifera, Scientific Results, 



Challenger Expedition. Zoology, XI. 

 1893. Woodward, A., and Thomas, B. Cret. Foraminifera, Minn., 



Neb. and 111. Minn. Geol. Surv., III., 28. 

 1898. Bagg, R. M. Cret. Foraminifera, N. J. Bull. U. S. G. S., 



no. 88. 



