AEGOCERAS LAT^ECOSTA. 365 



rather prominent ; ribs sharp, annular, elevated, inclined backwards, separated by wide 

 deep, concave spaces ; siphonal area convex, each rib as it passes across the area has a 

 slight elevation at the middle with an elevated line on the concave surface ; aperture 

 roundish, depressed, widest in the transverse diameter. 



Dimensions. — Transverse diameter 37 millimetres ; width of the umbilicus 20 milli- 

 metres ; height of the aperture 10 millimetres; transverse diameter 11 millimetres. 



Description. — This rare and beautiful little shell has sharp, elevated, and very 

 prominent ribs, as each of these passes over the siphonal area it is slightly thickened and 

 raised in the centre; the ribs are separated by concave spaces, the whorls being 

 narrower on the sides than on the area ; they diminish very gradually and regularly, " the 

 smaller whorls showing their prominent ribs like rows of jet beads;" hence Young called 

 it gagateus ; the ribs on the sides first incline backwards and afterwards arch forward before 

 they cross the siphonal area. 



This Ammonite resembles Aegoceras planicosta and Aeg. capricornus, but is very 

 distinct from both, and has a life in time intermediate between these two well-known 

 forms. The ribs are very prominent and the inner edge of the whorls much elevated, 

 sometimes overhanging the line of the spiral suture ; the whorls being only slightly 

 involute ; the spire is occasionally one-sided and a little distorted, when it very much 

 resembles Turrilites Cognarti, d'Orbigny. 



Affinities and Differences. — This species occupies an intermediate position in structure 

 between Aegoceras planicosta and capricornus. It wants the flattening of the ribs seen 

 in Aeg. planicosta, where they cross the siphonal area, and the spines which are developed 

 on their sides. The sharpness, narrowness, and elevation of the ribs distinguish it from 

 Aeg. capricornus, which it otherwise very much resembles. 



Localifg and StratigrapJiical Position. — Young's type specimens were collected from 

 the hard bands in the Alum-shale of the Hawsker shore. Tate and Blake state that 

 they obtained it from the zone of Ammonites oxynotus, of which it is highly characteristic, 

 at Robin Hood's Bay, Warter, and Market-Weighton. It appears to be absent from the 

 Lower Lias beds of the Midland Counties. 



Aegoceras lat^costa, Sowerby. PI, XXXII, fig. 1. 



Ammonites lat^costa, Sowerby. Min. Conch., vol. vi, p. 106, pi. 556, fig. 2, 1827.'^ 

 — BREVISPINA, d'Orbigny. Pal. Fran5. Terr. ; Juras8.,vol. i, p. 272, 1842. 



Diagnosis. — Shell discoidal, compressed, radiated ; whorls five, exposed ; radii large, 

 sharp, slightly waved, much flattened and widened as they pass over the rounded front ; 

 aperture oblong. 



^ In Sowerby's description of his plate 356 /or fig. 1 read fig, 2. 



