STEPHANOCERAS SUBARMATUM. 477 



plain rib passes over the back undivided, while the knobbed rib parts into three. The 

 double rib and knob resemble a button and loop, whence the name fibulatus, and some- 

 times the whole disk, from the margin of the back to the centre, is found marked in this 

 style." 



I have figured for comparison specimens showing the variation in the style of this 

 species, a variety in which the side knobs are small (PI. LXXXV, fig. 5), another 

 (PI. LXXXV, fig. 7) in which they are large and spinous, and another (PI. LXXXV, 

 fig. 8), which has received the name of Andrea ; in fact, this Ammonite presents many 

 modifications of its style of ribbing, but all the examples return into the loop and knob 

 style so well described by Mr. Young. PI. LXXXV, figs. 10, 11, is the var. Stephen. 

 Bollensis of Quenstedt. 



Affinities and Differences. — I have endeavoured to describe as accurately as possible 

 the peculiar character of the ribbing in Stephanoceras fibulatum, seeing that it is in this 

 character we discover the difference between it and Stephanoceras commune, and likewise 

 the distinction between Stephanoceras fibulatum and Stephanoceras subarmatum with which 

 it is very often confounded, as several specimens before me prove. 



Locality and Stratigraphical Position. — This species is found in the Upper Lias at 

 Whitby and Lofthouse, Yorkshire, in the zone of Stephanoceras commune. 



Stephanoceras subarmatum, Young. PI. LXXXV, figs. 1 — 4. 



Ammonites subarmatus, Young and Bird. Geol. Surv. of Yorks., p. 250, pi. 13, 



fig. 3, 1822. 



_ _ Sowerby. Min. Conchol., vol. iv, p. 146, pi. 407,fig. 1, 1823. 



Planites — Haan. Ammon. et Goniat., p. 84, 1825. 



Ammonites — Young and Bird. Geol. Surv., p. 263, pi. xiv, fig. 8, 1828. 



— — Simpson. Monogr. on Ammonites, p. 23, 1843. 



— — oVOrbiyny. Paleont. Fr., Terr. Jurass., p. 268, pi. 77, 1842. 



Diagnosis. — Shell discoidal, thick, whorls subquadrate, involute ; their width exceeds 

 the height ; transversely costated ; costse simple or fasciated ; inner whorls develop spiny 

 tubercles near the line of the spiral suture which are mostly absent in the last whorl ; at 

 the tubercle the costse split and divide into two or three finer costse which pass across 

 the margin ; area convex, flat, closely costated ; aperture less in height than in width ; 

 section subquadrate. 



Dimensions. — Transverse diameter 65 millimetres; height of aperture 20 milli- 

 metres; thickness 25 millimetres; width of umbilicus 30 millimetres. 



Description. — The shell of this species is thick, discoidal and compressed; the 



