ngb YORKSHIRE TYPE AMMONITES— II Dec. 



119. AMMONITES CRASSUS, Young & Bird 

 (Plate CXIX) 



Original Description 

 [Young & Bird, 1828, p. 253.] 



" In the hard bands in the alum shale, especially the ironstone bands, 

 we find a shell ribbed like A. communis, but much thicker in proportion 

 to its breadth ; the whirls being flattened transversely, and the aperture 

 of course being transversely oblong. The inner whirls diminish rapidly 

 in thickness, so as to form an umbilicus, or cup, more or less deep. The 

 ribs sometimes split into three on the back : the undivided part is usually 

 very short. This species, we have named A. crassus. There are two 

 or three varieties in the form, all essentially different from A. communis. 



Remarks 



Proportions, j * ^ 45. 46 j 



Subplaty-, perpachy- to pachygyral ; perlatumbilicate. 



Stages, conch, serpenticone ; periphery, i ; ornament, 5*. 



The species is craterumbilicate up to about 42 mm. diameter, then 

 tends towards planumbilicate. The costse are strong, hollow (septicostae) , 

 regular, mostly bifurcate at edge of periphery, rarely trifurcate, more 

 rarely a rib intercalated. 



Genus, Cceloceras, Hyatt ; family, Dactyloidae, Hyatt. 



Geological position : Simpson (1884, p. 86), says " I am not aware 

 that it is ever found along with A. communis, but lower in the series. . . . 

 U.L. i." 



Result 



Qeloceras crassum, Young & Bird sp. 1828, Whitbian, [sub- 

 carinatum (bifrons) zone], Whitby. 



