20 



GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



INTERAREAS 



According to Buckman/ the word area should be retained for general use, but interarea might be 

 used for "the area lying between the apex and the posterior line of valve-j unction — the cardinal 

 margin when there is a hinge, but at any rate the posterior margin." 



Interareas, or what were formerly called cardinal areas, are usual in most of the members of 

 the order Protremata and especially in its most primitive stock, the Orthoidea. True interareas are 



VEA/T/f'AL 





^j 



-■" 



Anacline OrthocUne ApsacUne CatacUne Proclcne 



(^ ^ 



fx 





f^ypercline 



An a c line Ort/iocCcne 



Apsacllne 



DORSAL 



Fig. 1. — Diagram to show various positions of the interarea with relation to the plane of the commissure. 



at times also well developed in the Telotremata, but never in the Atremata or Neotremata. When 

 a similar structure is present in the two last named orders it is due to holoperipheral growth or to 

 subsequent internal thickenings of the posterior margins, and in both these instances it is termed a 

 ■pseudointerarea. 



The interareas are, then, the plane or slightly curved surfaces of the palintrope, which is a 

 shelf, growing antero-ventrally or antero-dorsally according to the valve, and representing the pro- 

 gressive growth track of the hinge-line or plane of articulation. In describing or discussing the inter- 



Brach. Namyau beds, 1917 (1918), p. 453. 



