STEPHANOCERAS ANNULATUM. 475 



Stephanoceras anntjlatum, Sowerby . PL LXXXIV, figs. 7, 8, 9. 



Nautilus anguinus, Beinecke. Nautil. et Argonaut., fig. 73, 1818. 



Ammonites annulatus, Sowerby. Min. Conchol., vol. ii, p. 41, pi. 222, fig. 5, 



1819. 

 Planites anguinus, Haan. Amm. et Goniat., p. 89, 1825. 



Ammonites annulatus, Young and Bird. Yorksh. Coast, p. 253, pi. xii, fig. 11, 



1828. 



— — d'Orbigny. Paleont. Frar^., Terr. Jurass., p. 265, pi. 76, 



figs. 1, 2, 1842. 



— — Simpson. Monogr. on Ammonites, p. 21, 1843. 



— — Quenstedt. Cephalopoden, p. 173, tab. xiii, fig. 1 1, 1849. 

 Stephanoceras annulatum, Tate and Blake. Yorkshire Lias, p. 299, 1876. 



Diagnosis. — Shell discoidal, compressed ; whorls round, narrow, a little depressed on 

 the sides, and slightly involute; costse small, slender, flattened, annular, and very 

 numerous, some simple, others bifurcate, and both alternate ; siphonal area round, and 

 truncated beneath for the return of the spire. 



Dimensions. — Transverse diameter 80 millimetres; width of umbilicus 45 milli- 

 metres ; height of aperture 20 millimetres ; width of aperture 20 millimetres. 



Description. — The shell of Stephanoceras annulatum presents a marked difference to 

 the other forms of the Steph. commune group, like them it is discoidal and a little 

 compressed on the sides ; the whorls are narrow, round, and each revolution of the spire 

 is ornamented with one hundred and thirty slender costse, flat on the edge, and cut out 

 as neatly and sharply from the shell as if they had been chiselled with a tool (PI. 

 LXXXIV, fig. 7) ; the costse are all of the same size and directed obliquely for- 

 ward, some are simple and annular passing over the area undivided like rings, whilst 

 other bifurcate at the margin, so that the two forms alternate singly or doubly with one 

 another. Considerable irregularity in this arrangement, however, is sometimes visible, 

 and in those cases two or three simple costae alternate with one bifurcate rib. In early 

 life and up to a diameter of fifty millimetres, there was alternately one simple rib and a 

 bifurcate rib encircling the area, with increased dimensions ; when the shell had attained 

 to eighty millimetres in diameter, the number of bifurcated ribs began to increase and 

 the simple annular costae to diminish. The umbilicus is widely open and exposes all the 

 inner whorls of the spire, which consists of seven whorls a little higher than they are 

 wide ; all are slightly involute. The aperture is oval and only feebly grooved by the 

 return of the spire. 



Affinities and Differences. — Stephanoceras annulatum resembles Stephanoceras com- 

 mune (PI. LXXXIV, fig. 1), but is distinguished by its more numerous, slender ribs less 



