igb YORKSHIRE TYPE AMMONITES Mar. 



19. AMMONITES EXORTUS, Simpson 

 (Plate XIX) 



Original Description 



" 31. A[mmonites] exortus. [M. Simpson, 1855, p. 44.] 



["I. Without a dorsal keel or furrow. 



"a. No spines." p. 35.] 



" Volutions 5, exposed, outer whorl more than I the diameter ; 



radii prominent, strong, annular, separated by concave spaces ; aperture 



subquadrate ; diameter, i inch and three-tenths. 



"The aperture is nearly square, and on the outer angles of the 

 whorls the radii, in places, have slight tubercles. Mr. Clarkson's Col. 

 L.L. ; R. H. Bay [Lower Lias, Robin Hood's Bay]." [Simpson, 1684, 

 73 — the same.] 



Remarks 



Stages, conch, serpenticone ; periphery, i ; ornament, 4, passing to 5. 



Genus, Echioceras, Bayle, 1878 ; and in family arrangement 

 Echioceras should, perhaps, stand as a forerunner of the Hildoceratidae. 

 This species shows the stage of phylogeny in Echioceras before the develop- 

 ment of the carina — the ornament has attained stage 5 before carina 

 appears. 



Geological position would be, presumably, about Indurated Band 13. 



Of the specimens figured (Whitby Museum, No. 645), the larger one, 

 though slightly less than Simpson's dimensions, is reasonably concluded 

 to be his holot3^e ; the other a paratype. 



Result 

 Echioceras exortum, Simpson sp., 1855, Sinemurian [oxynotum 

 zone ?1, Robin Hood's Bay, near Whitby. 



