5b YORKSHIRE TYPE AMMONITES Dec. 



S. AMMONITES EXARATUS, Young & Bird 

 (Plate V) 



Original Description 

 [Young & Bird, 1828, p. 266.] 



" We have a smaller shell nearly corresponding with this [A. Mul- 

 gravius], but having the inner angle of each whirl sloping backward, 

 instead of projecting forward, and showing considerably less of the 

 inner whirls. This ammonite has also the grooved furrows more 

 uniformly marked ; and hence we have named it A. exaratus." 



Remarks 



No figure was given. 



Stages, conch, between serpenticone and oxycone ; periphery, 3c ; 

 ornament, 4c to 3c. 



There is a septicarina ; the umbilical edge is not undercut, but 

 steeply bevelled ; there is about half-a-whorl of body-chamber, the 

 specimen being, apparently, complete. There are the following stages 

 of development : — (i) the furcating rib stage in the umbilicus ; (2) the 

 single rib, falciform stage ; (3) the finer rib stage, less falciform. 



The species may be admitted into the genus Harpoceras, Waagen 

 (Gen p. 7'), but differs from the typical series of the genus by not having 

 an undercut umbilical edge, and by beginning to lose the strong falci- 

 form character of ribbing. 



The geological position, according to Simpson, is " U.L. 7, rather 

 abundant in the Jet Rock " (1884, 107). 



Result 



Harpoceras exaratum, Young & Bird, sp., 1828, Whitbian, 

 exaratum- zone, near Whitby. 



