474 THE LIAS AMMONITES. 



the convexity of which are directed towards the aperture. The spire is composed 

 of round whorls, a Httle higher than they are wide ; in some varieties the whorls incline 

 towards a quadrate section and embrace the penultimate whorl feebly so that this species 

 is extremely involute. The aperture exhibits nearly a round or sub-orbicular section 

 being hollowed out below to receive the return of the spire. In PI. LXXXIII, figs. 3, 

 4, I have figured a specimen of Stei^hanoceras commune from the British Museum 

 Collection which has the mouth aperture well preserved. There is first a circular 

 constriction, and then a round, prominent band surrounding the aperture ; there are no 

 side lappets ; and it is worthy of note that the mantle of the animal, when producing 

 this termination, formed the simple rib on the side and bifurcate ribs which extend 

 across the area. This specimen shows also that the dwelling chamber of Stepha- 

 noceras commune was very short and occupied little more than half the length of the last 

 whorl of the shell. The suture-line (PL LXXXIV, fig. 3) forms three lobes and three 

 saddles on each side. The siphonal lobe is long but wider than the principal lateral, and 

 develops on each side four digitations terminating in two pointed processes. The 

 siphonal saddle is much larger than the principal lateral lobe, and terminates in 

 three unequal, trilobed folioles. The principal lateral lobe has an irregular figure, 

 having three digits on each side and a terminal obtuse one. The lateral saddle is small 

 and oblique, terminating in four unequal folioles. The inner lateral lobe is small and 

 oblique, and terminates in two points, the auxiliaries are quite rudimentary. 



Locality and Stratigraphical Position. — This Ammonite, as its name implies, is one 

 of the most common fossils in the Upper Lias, and vras at one time found in great 

 numbers near Whitby, some parts of the beach at one time were literally paved with 

 nodules containing them ; in those days a cart load might have been had in a short time ; 

 the fossils were usually inclosed in nodules and it was for these the search was made, 

 as the nodules were largely used in the manufacture of Roman cement ; the demand 

 having exceeded the supply, they are now no longer abundant. 



Although this species is so common in England, Prof. d'Orbigny stated that he did 

 not know it in France ; however, since that time, Dumortier has figured a very good 

 example from the Ironstone Sands at la Verpilliere, Isere ; where it is associated with 

 Stephanoceras annulatum. 



It is very commonly met with as a leading fossil in the Upper Lias strata of 

 Northampton, Warwick, Gloucester, Somerset, and Dorset, and has a wide distribution 

 in these counties. 



