314 THE LIAS AMMONITES. 



outer margin, and which pass over the siphonal area in a series of arched lines, their 

 convexity being always directed towards the aperture in Aeg. intermedium (figs. 4, 5, 6). 

 Locality and Stratigraphical Position. — The two specimens I have figured were 

 collected from the Lower-Lias zone of Aeg. planorbis, North Cliff, at Robin Hood's Bay. 

 Mr. Blake notes this species under the name of A. Johnstoni from Coatham Marsh, and 

 from the zone of Aeg. angulation, Cliff, Leigh Dam, Scar, Eston Pit, Yorkshire. I have 

 not found this species in any of the collections of Lias fossils in Gloucestershire. 



Aegoceras intermedium, Portlock. PI. XV, figs. 3 — 6. 



Ammonites intermedius, Portlock. Geology of Londonderry, p. 136, fig. 17, 1843. 

 _ _ Morris. Catal. British Fossils, p. 292, 2nd ed., 1854. 



Shell discoidal, compressed ; whorls narrow, numerous, seven to eight exposed and 

 slightly involute, flattened on the sides and rounded on the outer margin ; ribs acute, 

 straight, or slightly bent, enlarged at the outer margin, where they bend round and 

 disappear from the rounded siphonal area ; shell with delicate strise, which bend forward 

 and meet over the front. 



Dimensions. — Diameter 70 millimetres ; width of umbilicus 42 millimetres ; height 

 of the last whorl at aperture 12 millimetres ; width of ditto 12 millimetres. 



Description. — This Ammonite was first figured by General Portlock 1 in his valuable 

 and exhaustive report, where it is thus described : — " Section oval, the axis in the 

 plane of the larger whorls being about one-fourth longer than the transverse axis ; 

 ribs not extending to the suture on the inside nor over the front on the outside. 

 When the shell is preserved there are very delicate, almost obsolete strise, which 

 bend forward and meet over the front. The front itself to the eye is quite smooth, 

 but occasionally some irregularity may be discovered by the finger. Ribs are visible 

 almost at the centre, and are sharper and proportionately longer in the inner whorls ; 

 occasionally, also, they are slightly bent. This species differs from A. communis 

 in not having forked ribs ; from A. jjlanicosta, in not having the ribs sharply 

 bevelled over the front, though in some specimens traces of a continuation of obsolete 

 ribs over the front may be observed, probably constituting another intermediate variety ; 

 from A. Conybearii in not having a keel ; and from A. Johnstonii, to which it closely 

 approximates, in having the ribs closer together and proportionately longer, though in 

 this latter respect it may be observed that, Sowerby's description having been drawn up 

 from a flattened specimen, the breadth of the volution will necessarily appear greater in 

 proportion to the ribs than in the natural condition. The foliaceous septa occupy the 

 space between two ribs, and are extended in broad digitate lobes over the front. 

 1 ' Report on the Geology of Londonderry, pp. 136 — 8, fig. 17, 1843. 



