476 THE LIAS AMMONITES. 



regularly bifurcated, more closely placed together and neatly cut as with a tool, and in 

 addition by its annular, straight ribs which are not bent forward, at the point of inflection. 

 Locality and Stratigraphical Position. — This species is found in ironstone nodules in 

 the Upper Lias of Whitby in the zone of Stephanoceras commune. 



Stephanoceras fibulatum, Sowerby. PI. LXXXV, figs. 5 — 11. 



Ammonites fibulatus, Sowerby. Min. Conchol., vol. iv, p. 147, pi. 407, fig. 2, 



1823. 



— — Young and Bird. Geol. Surv. York. Coast, p. 262, pi. xiii, 



fig. 9, 1828. 



— Simpson. Monogr. on Ammonites, p. 22, 1843. 



— Andrew — Yorkshire Lias, p. 59, 1855. 

 Stephanoceras fibulatum, Tate and Blake. Yorkshire Lias, p. 301, 1876. 



Diagnosis.— Shell discoidal, compressed; whorls round on the area, flattened on the 

 sides, and slightly involute; spire entirely exposed; costae numerous, united in pairs by 

 a small, smooth tubercle near the outer margin, then split again into two or three arched 

 costae on the area where they join their fellows. Section of aperture subquadrate. 



Dimensions. — Transverse diameter 85 millimetres ; breadth of area 20 millimetres; 

 height of aperture 2:2 millimetres ; width of umbilicus 46 millimetres. 



Description. — One of the most obvious characters in this very beautiful Ammonite 

 is the uniting of the radii by knobs, like a button and loop ; in some specimens this 

 style is very uniform, in others modifications are observed ; for example, in one shell 

 two of the radii unite on the knob, and then a single rib passes up between them, 

 bifurcates and extends over the area ; in other examples three ribs are seen to be 

 united. The whorls are flattened on the sides and ornamented with upwards of fifty 

 sharp, oblique costae (PI. LXXXV, figs. 6 and 7) which commence at the spiral suture, 

 ascend obliquely upwards and forwards to the border of the siphonal area, where a 

 spiny tubercle is developed, from this point two small costae arise and cross the area 

 in an arched form, the convexity being directed towards the aperture. The siphonal 

 area is round and highly ornamented by the ribs which cross it with great regularity. 

 The costae on the inner whorls are usually small, so that within the umbilicus the 

 shell is in general fine-ribbed, presenting a marked contrast in this respect to the 

 highly ornamented sides of the last whorl. Mr. Young, who had a large series of 

 this species in his possession, says in his ' Survey of the Yorkshire Coast,' " Some of 

 the inner whorls are marked with double ribs meeting in the knobs, and then parting 

 into three on the back ; and on the outer whorl or whorls, we find strong, single ribs, 

 knobbed, each splitting into three beyond the knobbed towards the back. In some 

 specimens, the ribs on the outer whorl are alternately knobbed and plain ; in each case the 



