n 3 b YORKSHIRE TYPE AMMONITES—I I July 



113. AMMONITES ROTIFER, Williamson-Brown 

 (Plate CXII1) 



First Mention 

 [Williamson, Distr. Org. Rem. Yorksh. Coast ; Trans. Geol. S. (2) 

 VI, 1841, 147.] 



" Kelloways Rock 



" This is best exposed at Gristhorpe, Cayton Bay and Scarborough 

 Castle Cliff. At the latter place it presents the following ascending 

 section : — 



(a) Soft variable beds 35ft. 



(b) Hard ironstone 6ft. 



" The ironstone (b) is full of fossils .... the greatest bulk 

 . . . . found towards the upper portion of the main mass of the 

 stratum. The following .... most abundant : 



" Ammonites ichthyodorsus (nob.) 



— gamma (nob.) 



— rotifer (nob.) 



— oblisus (nob.) " 



Original Description 

 [T. Brown, 1849, P- 2 4b-] 



" 21. Ammonites rotifer. — The Wheel-like Ammonite, pi. XX*. 

 figs. 14, 15. 



" Discoidal, with five moderately rounded, smooth volutions, which 

 are one-third concealed ; a series of close-set ribs pass over the rounded 

 back, and reach nearly the middle of the volutions on either side, where 

 they are met by wide-set, thick ribs, which emanate from the inner 

 margins ; aperture nearly orbicular. 



"The Calcareous Grit, Scarborough. 



" In the Manchester Museum." 



Remarks 



Proportions, 53, 27, 40, 48 ; subplaty-, pachygyral ; perlatum- 

 bilicate. 



Stages, conch, serpenticone ; periphery, 1 ; ornament, 4c ( — 4, 

 tending to renewed anagenesis ?). 



Whorls depressed, sides gibbous, greatest thickness about middle ; 

 no definite inner edge ; ribs commence at line of contact, and are not 

 hooked. Periodic constrictions, 2 to a whorl, strong. On flatly arched 

 periphery the slightest sign of a median interruption of ribs (specimen 

 without test). All ribs of low relief. Primaries slightly ( shaped, 

 breaking on edge of periphery into, first, on outer whorl, 2, later, 3 

 secondaries, with another rib generally intercalated — primaries becoming 

 more pronounced with age. The secondaries have slightly sinuous course 

 across periphery with, except towards end of whorl, slightest median 

 bow backwards. 



Suture-line, EL longer than L x ; auxiliary series just longer than 

 EL; L z rather small, L 3 more developed than L 2 - and very oblique. 

 Specimen wholly septate, without test on outer whorl. 



