AEGOCERAS LECKENBYI. 345 



is consequently ornamented with numerous close-set transverse folds, nearly uniform in 

 thickness throughout. The lobe-hne is extremely convoluted. 



Dimensions. — Transverse diameter, 85 millimetres ; diameter of umbilicus 35 milli- 

 metres ; height of last whorl 30 millimetres; transverse diameter 30 millimetres; 

 amount of involution one third the height of the whorl. 



Description.— The rounded whorls with small ribs and blunt elongated tubercles, 

 having the siphonal area crossed by numerous transverse folds, and corresponding valleys 

 of about the same proportionate development throughout, form a group of characters which 

 distinguish this species from its congeners. The number of primary lateral ribs varies in 

 the two specimens here figured from fourteen to twenty-four; they arise near the 

 spiral suture by a thickened fold, and, describing a sigmoidal flexure, develop near the 

 margin of the siphonal area a blunt elongated tubercle, from which two or three smaller 

 folds proceed across the area; the tubercles are neither large nor prominent (figs. 1, 2, 

 4, 6), and do not terminate in spines, at least in the specimens I have seen. The 

 secondaries are smaller annular folds, which arise from the spiral suture, describe a 

 sigmoidal flexure, and extend directly across the siphonal area, blending with their fellows 

 from the opposite side, and covering the area with a series* of elevations and depressions, 

 which impart an ornate aspect to this region of the shell (figs. 2, 3, 6). The whorls are 

 quite one-third concealed in the umbilicus by the involution of the spire (figs. 1,4). 



The lobe-line, as in all the other Armati group, is extremely convoluted (fig. 7) ; the 

 siphonal lobe is long, and has one large lateral and three smaller terminal branches. The 

 siphonal saddle presents three deeply ramified folioles ; the principal lateral lobe is as large 

 as the siphonal, and divides into two branches, which are both much digitated. The 

 lateral saddle is small, narrow, and foliated, and the accessary lobes have a central stem 

 with lateral digitations. The whole forms an oblique and most complicated line of 

 suture, very difficult to trace through all its ramifications. 



Affinities and Differences — This species very much resembles Aec/. armatum, but 

 difiers from it in the following details of its anatomy. The whorls are rounder and more 

 involute ; the spines are shorter, elongate, and non-aculeate ; the lobe-line is much less 

 convoluted, and the transverse folds across the siphonal area are smaller and more 

 numerous. 



Locality and Stratigraphical Position. — This Ammonite was obtained in the Arraatum- 

 zone of the Middle Lias at Lyme Regis, associated with Ae^. armatum and Aeg. miles, 

 and the other fossils of this bed. It must be a rare form as I have not seen any other 

 specimens except those here figured, nor observed it in any of the collections examined 

 for the purpose of acquiring all the diflFerent species of Armati for my work. This 

 species is dedicated " in memoriam " to my old and much esteemed friend, the late Mr. 

 John Leckenby, F.G.S., of Scarborough, who was always a ready and willing helper by 

 the loan of his most beautiful specimens for figuring in my works on the Eossil Echino- 

 dermata and Ammonitid^, published by the Palseontographical Society. 



45 



