THE TEST 13 



cave in forms which are attached by the dorsal side. That por- 

 tion of the chamber which is adjacent to or contains the aper- 

 ture is spoken of as the apertural face. 



Chambers are usually simple (PI. 29, fig. 6), that is undivided. 

 In many of the higher forms they are divided into chamberlets 

 (PI. 32, fig. 5). In some forms, especially among the arenaceous 

 group, there is a secondary filling of chambers resulting in 

 "labyrinthic" chambers (PI. 10, fig. 15) . This is usually the sign 

 of the approaching culmination of a group as are similar struc- 

 tures elsewhere. 



In the orbitoid group and in some of the other large forms, 

 the proloculum and immediately succeeding chambers are spoken 

 of as the "nucleoconch". 



In this group also there are two distinct sets of chambers de- 

 veloped, those of the central plane known as equatorial cham- 

 bers and those of the two sides known as lateral chambers. The 

 shape and position of these make important diagnostic charac- 

 ters in the orbitoids. 



In different descriptions .there is a great discrepancy in the 

 use of the words height, breadth and thickness as applied to 

 chambers. It may be convenient to think of an uncoiled form 

 such as Saracemaria (PI. 24, fig. 3) as a simple one. If the test 

 is oriented with the aperture up, the height and breadth of the 

 chamber will be as ordinarily understood and the thickness, the 

 measurement generally at right angles to the breadth. If the 

 straight forms are for the most part uncoiled forms then the 

 coiled forms may be similarly treated. The height of the cham- 

 ber will be the distance between the sutures generally in the 

 axis of coiling, and the breadth the measurement at right angles 

 to the coiling or between the periphery and the spiral suture. 

 This terminology will allow of the same relative use in coiled 

 and uncoiling chambers which often occur in the same species. 



Specialized chambers are sometimes developed at certain 

 stages of the individual, illustrations of which are the "float" 

 and spherical chambers developed in the adult free-swimming 

 stage of Tretomvhalus (PI. 43, fig. 10). 



Sutures. The division between chambers and between whorls 

 are spoken of as sutures. The line between the succeeding 

 whorls or coils in a test is often spoken of by authors as the 

 spiral suture in distinction from the ordinary sutures between 



