PHYLLOCERAS SUBCARINATUM. 429 



species has not been included in the classical works on Liassic Cephalopoda by 

 Quenstedt, d'Orbigny, and Von Hauer, nor has it been noticed as a doubtful species, or 

 one that had fallen under their observation in any way ; — an omission which may perhaps 

 be explained by the fact that it was very badly drawn in the two Yorkshire mono- 

 graphs in which it was first figured. Dr. Oppel correctly described this species and 

 pointed out its affinities in his Juraformation, as he had obtained the fossil himself at 

 Whitby from the dealers and showed me the purchased specimens. One of these he has 

 figured most beautifully in his ' Palaontologische Mittheilungen,' pi. 44. 



The shell is extremely thin and ornamented with numerous fine annular striae, which 

 pass over the area and encircle the whorl. The siphonal area is wide and has an obtuse 

 ridge in the centre and two flat bands extended on each side of the carina, but without 

 any groove or sulcus, such as exists on each side of the keel in the genus Arietites. 

 The umbilicus is narrow and the inner whorls are much concealed (PI. LXXXI, fig. 1). 

 The aperture is ovate (PI. LXXXI, fig. 2), widest across the inner or spiral side, and 

 gradually tapering from thence towards the siphonal area (PI. LXXXI, fig. 2). This 

 area is extremely characteristic, having an obtuse ridge running round the middle line of 

 the shell, with two lateral flat bands on each side (PI. LXXXI, figs. 2 and 3), forming 

 a structure by which this species is readily distinguished from all its congeners. 



The suture-line is not exposed in a satisfactory manner in any of my specimens, 

 the thin shell being so often retained in the matrix that the suture-lines cannot be 

 traced. 



Affinities and Differences. — The Ammonite which most nearly resembles Fhyll. 

 suhcarinatum is the PJii/lL cy chides, d'Orbigny, from the Inferior Oolite of Moutiers. 

 The young forms of this species have a keel, lateral ribs, and a narrow umbilicus, but in 

 later hfe the shell becomes smooth, the keel is less prominent, and the lateral bands are 

 smoother, the ribs disappearing ; with all its affinities Phyll. cycloides is, nevertheless, a 

 very different form to Phyll. suhcarinatum. 



Locality and Stratiyraphical Position. — It is obtained from the Alum-shale, near 

 Whitby, in company with Harpoceras bifrons, Harp. Lythense, and Stephanoceras com- 

 mune. This species is rare when compared with the other forms with which it is 

 associated. 



Dr. Oppel states that in the Munich Museum there is a specimen, named by Miinster, 

 from Altdorf, in Franconia, out of the J/o^o^zs-limestone ; and in the Berlin Museum 

 there is a specimen from the Iron-earth of Verpilliere, near Lyons ; so Phyll. suhcarinatum 

 is a leading fossil for the Upper Lias in France and Germany, as well as in the British 

 Isles. 



