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part of the same, between the " lateral angles" or the "umbilical 

 shoulders." The "zone of impression," or '-impressed zone," is 

 the area on the dorsum, which is concave, and lies between the 

 "umbilical zones." The impressed zone may appear indepen- 

 dently as a "dorsal furrow," or, after contact, as a " contact fur- 

 row." The " zone of inclusion "or " included zone " is the cov- 

 ered area corresponding to this on the. venter. The term "zones 

 of involution" or "area of involution" can be used for both of 

 these when the whorls are not separated or it is desired to speak of 

 the two together. The " lines of involution " are the outer bound- 

 aries of the "zone of impression " on the dorsum, and the "lines 

 of inclusion " the corresponding lines on the venter. 



The terms "involved" and "involution " should be limited to 

 whorls having a "zone of impression " or " impressed zone," that 

 is, to "nautilian" shells. " Coiled " can be applied to all shells 

 that have the gyroceran curve and even to shells with the whorls 

 in contact. Nevertheless these sometimes have closer affinity with 

 nautilian shells of a given series than with the gyroceran shells of 

 the same series. 



Whorls with only two surfaces and angles are "digonal ;" three 

 surfaces and angles "trigonal;" four surfaces and angles "tetra- 

 gonal," and when the abdomen is much broader than any other 

 side "trapezoidal;" five surfaces and angles "pentagonal;" six 

 surfaces and angles " hexagonal ;" seven surfaces and angles " hep- 

 tagonal;" eight surfaces and angles "octagonal ; nine surfaces and 

 angles "enneagonal ;" ten surfaces and angles "decagonal." 



The outlines on the Diagram Plate, opposite, page 425, will be 

 found to explain these terms more fully. 



Explanation of Diagram Plate. 



Diagram A. — Section of compressed elliptical whorl with primi- 

 tive regions indicated, ananepionic substage of nautilian shells and 

 ephebic stage of many primitive orthoceran and cyrtoceran forms. 



Diagram B '. — Section of depressed elliptical whorl occurring 

 older in the ontogeny or correspondingly later in the phylogeny 

 than A. 



Diagram B" . — Section of a reniform whorl with a contact fur- 

 row. This may be evolved from B' by the growth and involution 



