12 



FORAMINIFERA 



types by division into chambers are derived many forms in the 

 different groups. From the conical spiral types have developed 

 great groups such as the Rotaliidae and the families derived 

 from it. Such forms are spoken of as trochoid tests, after the 

 genus Trochus in the Mollusca. In such forms the chambers of 

 all the whorls are visible from the dorsal side but usually only 

 those of the last formed chamber from the ventral side (Fig. 

 1). There is usually an umbilical area in the central portion 

 of the ventral side which may be either open or variously filled. 



Peripheral riargm 



Aperture Apertural Face 



AperTure Urn(ji|icua 



Spiral Suture 



Outurea 



FIGURE 1 



SuTiAres 



bate SuTureb 

 FIGURE 2 



Chambers. The initial chamber in the foraminifera is known 

 as the proloculu7n and may be either small and the result of the 

 union of zoospores — the tnicrospheric proloculum — or larger and 

 the result of asexual division — the megalospheric proloculum. 

 In primitive forms the second chamber is often elongate and 

 tubular, either straight or variously coiled. Chambers may be 

 closely coiled, loosely coiled, or uncoiled. In many coiled forms 

 the chambers remain evolute, showing those of the earlier coils 

 as in Assilina (PI. 29, fig. 7) or closely involute, covering the 

 earlier coils as in Camerina (PI. 29, fig. 15) . Many of the terms 

 used in the description of chambers need no explanation as they 

 are common terms and easily understood. The portions of the 

 chamber are named similarly to those of the walls and are usu- 

 ally thought of in connection with the wall. Such terms as 

 peripheral, proximal, dorsal, ventral, etc., will be readily under- 

 stood. In trochoid forms (Fig. 1) the side which shows all the 

 whorls of the spire is the dorsal side, and the one showing only 

 the last-formed whorl is the ventral side. The dorsal side is 

 usually more or less convex but may be flat or even slightly con- 



