77 b YORKSHIRE TYPE AMMONITES August 



yy. AMMONITES REGULARIS, Simpson 

 (Plate LXXVII) 



Original Description 

 " 159. A[mmonites] regularis. [M. . Simpson, 1855, P- 89.] 



[" II. With a keel on the back/' p. 72. 

 " b. Outer whorl narrower." p. 8j.] 



" Volutions 4, inner ones J concealed, outer whorl more than J the 

 diameter, with the sides rounded ; radii numerous, regular, straight, 

 bend towards the aperture near the keel ; keel depressed ; aperture 

 acutely triangular, or ovate ; diameter 1 inch and two-tenths. 



" This ammonite is much more depressed than A. geometricus, 

 the outer whorl is wider, and the radii are obtuse, and less numerous ; 

 the shell is roughish, and without striae. I believe it is from the lower lias 

 at R.H. Bay. — Mr. Clarkson's Col." 



Simpson, 1884, p. 127, the same. 



Remarks 



Proportions, 32, 40, 19, 29 ; platy-, subleptogyral, sublatum- 

 bilicate. 



Stages, conch, platycone ; periphery, 4c, carina crenulate ; orna- 

 ment, 4c. 



About half a whorl is body-chamber. The crenulate low keel is 

 bordered by a slight depression each side, presumably obsolescent furrows. 

 The radii are straight on the lateral area, and have indications of swellings 

 (perhaps obsolescent tubercles), where they turn forwards on the 

 peripheral margin. Peripheral projection not very pronounced, but 

 difficult to follow. 



The specimen is not in good preservation, and the middle is 

 damaged. 



Genus, Paltopleuroceras, S. Buckman, presumably a dwarf, possibly 

 a cripple, at any rate decrepit ; family Amaltheidae. 



Geological position, matrix, marly oolitic, with a greenish tinge, 

 the grains small, crowded and white : perhaps Simpson's " indurated 

 grey band," Mid. Lias a (1884, xv). 



Result 

 Paltopleuroceras regulare, Simpson sp. 1855, Domerian, [acutum 

 or spinatum zone ?] Robin Hood's Bay, near Whitby. 



