PROTOZOA. FORAMINIFERA 



25 



Nummulites. (figs. 2, 4.) Shell lenticular in form, and com- 

 posed of a large number of whorls coiled In a plane spiral. Usually 

 each whorl completely covers the preceding one by means of the 

 lateral prolongations of the chambers, so that externally only the 

 last whorl of the shell is visible. The whorls are divided into 

 chambers (c) by septa (b) which are slightly curved backwards ; 

 each chamber communicates with the neighbouring one by means of 

 a median fissure at the inner margin of the septum. Each septum 

 is formed by two lamellae. A supplemental skeleton is present, 

 part of it forming what has been termed the ' marginal cord ' (a). 

 The general shell-substance is minutely perforated, and a system of 

 canals also traverses the septa and supplemental skeleton. Aperture 



a, mar- 



Fig. 4. Nummulites, showing vertical and horizontal sections. 



ginal cord with canals (supplemental skeleton); b, septum, with 

 canals ; c, chambers ; d, test ; e, pillars of the supplemental skele- 

 ton. (After Zittel.) Enlarged. 



in the form of a slit at the inner margin of the last chamber. The 

 shell splits readily into two similar parts along the median plane, 

 owing to the relatively large size of the parts of the chambers 

 occurring there. The earliest species of Nummulites occurs in the 

 Carboniferous Limestone of Belgium ; others have been recorded 

 from the Upper Jurassic of Amberg (Bavaria) ; the genus attains 

 its maximum in the Eocene ; only one or two rather rare forms are 

 living, one of which (N. cummingi) is found in shallow water in 



