82b YORKSHIRE TYPE AMMONITES— II August 



82. AMMONITES RETUSUS, Simpson 

 (Plate LXXXII) 



Original Description 

 " 85. A[mmonites] retusus. [M. Simpson, 1855, p. 62.] 



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

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



" Rather depressed ; volutions 6, exposed ; radii obtuse, annular, 

 split in two in passing over the rounded back, in the middle of which 

 they have a slight bend towards the aperture, sometimes obsolete on 

 the inner whorls ; on the outer whorls, towards the outer margin, an 

 irregular row of tubercles ; aperture nearly round, rather transverse ; 

 diameter 2 inches. 



" The shell, which is seldom preserved, is thin, and without striae ; 

 the tubercles are strong and pointed, and come off with the shell, leaving 

 their flatted bases on the cast. The ramifications of the septa, with their 

 fingered and bushy lobes, are more numerous than in the last 

 [A. marshallani, Simp.]. — L.L. ; R.H. Bay." 



Additional Details 



Simpson, 1884, 94, 95. — p. 95 has at end of first instead of 2nd par. 

 " L.L., y [y] R.H. Bay." 



Remarks 



Proportions, 54, 31, 37, 46 ; subplaty-, pachygyral, perlatum- 

 bilicate. 



Stages, conch, serpenticone ; periphery, 1 ; ornament, 5*. 



The early whorls are slowly coiled, and are almost smooth up to 

 about 15 mm. diameter. Then a rib-stage begins but is not pronounced ; 

 and the first sign of tubercles appears at about 18 mm. diameter ; but 

 it is difficult to say exactly, for there remain no more than indistinct 

 scars of basal septae. The tuberculate stage is intermittent, especially 

 at first : there being smaller plain ribs between larger tuberculated ribs, 

 but later these smaller ribs show small scars, as if they had' carried septate 

 spines. If so, the tuberculation must have been irregular in size, as well 

 as irregular in occurrence (intermittent). The smaller ribs are mainly 

 single, the larger tuberculate ribs bi- or trifurcate at the tubercle. 



The periphery is somewhat flattened ; the sides of the whorls are 

 divergent, though convex. 



Genus, Deroceras, Hyatt ; family Deroceratidae. 



Result 



Deroceras retusum Simpson sp. 1855, Charmouthian, armatum 

 zone, Robin Hood's Bay, near Whitby. 



