7b YORKSHIRE TYPE AMMONITES Dec. 



7. AMMONITES DENNYI, Simpson 

 (Plate VII) 



Original Description 



"7. A[mmonites] Dennyi. [M. Simpson, 1843, pp. 9, 10.] 



[" I. Without a dorsal keel or furrow. 

 " a. No spines." p. 7.] 



" Volutions 3, inner ones J concealed, outer whorl about J the 

 diameter ; sides flatted, back obscurely triangular ; radii few, nearly 

 obsolete ; sulci or constrictions regular, small, distant, straight, diverging, 

 distinct on the inner margin of the whorl, obsolete on the back ; aperture 

 subquadrate ; diameter i inch. 



" This small Ammonite is from the lower lias at Robin Hood's Bay. 

 Considering its small size, it is rather a thick species. The [p. 10] 

 constrictions are generally obsolete on the back, but in some places 

 they are visible ; they are then observed to make a sudden bend towards 

 the aperture. It is smooth and shining, and of a bronze colour. 



" In naming this species, I have great pleasure in the recollection 

 of Mr. H. Denny, Keeper of the Leeds Museum, whose pencil has long 

 been employed in illustrating the minutest part of animated nature, 

 and whose talents and devotedness to Natural Science do honour to 

 the institution with which he is connected." 



Additional Details 



Simpson, 1855, 38, has, instead of 3rd par., " Var. without con- 

 strictions " ; 1884, 66, in ist par., after " triangular," has " or round " ; 

 after "subquadrate " has " or ovate " ; adds to 2nd par., " ind. band 3." 



Remarks 



Stages, conch, spha;rocone, and incipient oxycone ; periphery, i ; 

 ornament, i and incipient 5. 



The septal margin is extremely simple, there being only a Goniatitic 

 lobe-line, with serrated edges. The sides show L' and L^, both very short. 



Of the three examples figured, i and 2 agree with Simpson's measure- 

 ment ; but fig. I seems to agree best with his description : therefore 

 it is presumably the holotype. 



The general appearance of the specimens suggests the genus Cym- 

 bites, Neumayr ; but in fig. i, marked by the arrow, is an auriculoid, 

 a specialized tuberculate development (Introd., p. ix), which may be 

 marked as sj. Quenstedt illustrates how .-1. riparius, Oppel, which 

 is in stage 5, and might be reckoned as an Agassiceras, develops into 

 A. auritulus, stage 5! (Amm. Schwab, xxiv, 13 — 16). Agassiceras 

 shows the development of stage 2 into 5 (see No. 6). But A. dennyi 

 develops stage i into 5! direct. However, the auriculoid shows that 

 A. dennyi must be descended from a form that had reached stage 5j, 

 e.g., A. auritulus. A. oxynotus, which Pompeckj has rightly noted as 

 a descendant of riparius ( + auritulus), is even more post-auritulum 

 than A. dennyi, for in its ontogeny it often omits the auriculoid stage : 

 it passes from i to 3c direct, though its ancestors have come through 



