834 



ERICSON 



[chap. 31 



iiseful are Globorotalia trimcatulinoides, G. hirsuta and Globigerina pachyderma. 

 These species construct fairly robust tests which sometimes occur where the 

 tests of many other species have been completely destroyed. In designating the 

 mirror image forms as right or left the gastropod convention has been followed, 

 that is the test is oriented with the dorsal or aboral side up. If when so viewed 

 the chambers have been added in a clockwise sense or cum sole, the test is called 

 dextral, and vice versa. In counting the tests we have found it convenient to 

 use two tally counters. When the species is abundant we continue to count 

 until one hundred dextral or sinistral tests have been counted. This takes from 



'°| COILING OF Globor alalia Iruncalulinoides 

 IN SURFACE LAYER OF CORES FROM 

 THE NORTH ATLANTIC 



67% 



20 



88% 



Right 



/ 62% . 

 /' • 



. ' -'Left 



'•* 77%_^* •*84% ,^^. 

 ^--^rw \ '4/ot?-^^Conaries Co 



■ • \ 



86% •_• 

 Right 



Equotorlol Suite 



\vp.-2774j2' 100% Right 



VI4-5 



76 



73 



20 



Fig. 1. Chart of coiling of Glohorotalia truncatulinoides in the surface layer of sediment in 

 the North Atlantic. 



one to two minutes. The coiling ratio may be expressed either as a percentage of 

 the dextral tests in the total count or as a percentage of tests of the dominant 

 coiling direction in the total count. 



Curves of variation in coiling direction have been used successfully in the 

 correlation of oil wells by Bolh (1950, 1951), Vasicek (1953) and Nagappa (1957). 

 However, the method can be used only where planktonic species are common. 

 Although chiral symmetry is frequent among benthic species, dextral and sinis- 

 tral tests are normally about equally abundant. Apparently the geometry of the 

 tests plays a critical role only in the planktonic way of life. 



