84b YORKSHIRE TYPE AMMONITES— II Nov. 



84. AMMONITES NATIVUS, Simpson 

 (Plate LXXXIV) 



Original Description 

 "99. A[mmonites] nativus. [M. Simpson, 1855, p. 68.] 



["I. Without a dorsal keel or furrow." p. 35 

 " b. Armed with spines or distinct tubercles." p. 58.] 



" Volutions 6, exposed ; outer whorl more than J the diameter, 

 inner margin rounded, prominent, slightly overhanging the succeeding 

 one ; radii obtuse, irregular, more numerous on the outer whorl, annular, 

 a row of blunt tubercles on the outer half of the whorl, more distinct 

 towards the aperture, the radii connected with the tubercles split in two 

 on the back, between these are from one to three radii which pass over 

 the back undivided ; aperture circular ; diameter 2 inches. 



" As far as I can judge, the shell is smooth. The ramification [s] 

 of the septa are very numerous, delicate, and angular, occupying nearly 

 the whole whorl. This specimen, and a fragment of a larger one, are 

 much cracked, and very fragile, arising apparently from the abundant 

 pyrites they contain. — L.L. ; R.H. Bay." 



Simpson, 1884, p. 101, the same. 



Remarks 

 Proportions: | £ g g 45} 



Subplaty-, pachygyral, perlatumbilicate. 



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



The whorl section is nearly circular. 



The tubercles are intermittent : 2 or 3 plain ribs may occupy the 

 spaces between the tuberculate ribs. On the last half-whorl shown 

 there are 6 septate tubercles or signs of tubercles, of which the last 3 are 

 separated by about half-inch spaces ; while there are some 17 ribs. At 

 the tubercle there may be bi- or trifurcation ; there is also occasional 

 bifurcation where the rib is plain. 



The tuberculate stage seems to begin early, after a short smooth 

 stage : there may be a short regular coronate (tuberculate) stage directly 

 following the smooth stage ; but the specimen is not in good condition. 

 A part of the specimen — not photographed — is detached, and is nearly 

 destroyed by decomposition of pyrites. 



Genus, Deroceras, Hyatt ; family, Deroceratida?. Geological position, 

 presumably L.L. y. 



Result 

 Deroceras nativum, Simpson sp. 1855, Charmouthian, [armatum 

 zone], Robin Hood's Bay, near Whitby. 



