356 THE LIAS AMMONITES. 



Rev. J. E. Cross, F.G.S., for the loan of some large specimens of this Ammonite, one of 

 these I have figured in PL LII, two-thirds the natural size, and I have another of much 

 larger dimensions in my custody ; it therefore attained a gigantic size, varying very little 

 from the type figured. 



The volutions are uniformly rounded, and about one-third or one-fourth of the width 

 of a whorl is involute ; the outer whorl in height is one-third the diameter of the shell, its 

 sides are convex, and ornamented with twenty-five narrow, slightly bent ribs, which dis- 

 appear in some shells near the margin of the area (PL LII, figs. 1 — 2), and in others 

 extend across the space (PL LIIa, fig. 2). In such shells an elevated ridge is developed on 

 the mesial line and runs longitudinally round the whorl, connecting the ribs with one 

 another, as shown in PL LII, fig. 5, and PL LIIa, figs. 2, 4, and 6 ; and the ribs are 

 separated by concave spaces three times the width of the ribs. 



In most of the young shells which have passed through my hands, measuring from 50 

 to 60 millimetres in diameter, the ribs terminate in a thickened portion at the margin 

 of the area, whilst the intervening space is smooth, as in PL LII, fig. 4 and fig. 5 ; 

 in other specimens a central rudimentary carina distinctly shows itself, as in PL 

 LIIa, fig. 2, and figs. 4 and 6. This is a true Aeg. Jamesoni character of early life. See 

 PL LI, figs. 5 and 6, in which specimen it is very well seen. The shell is partially 

 preserved in some of my smaller examples, which enables me to state that it is extremely 

 thin. 



The umbilicus is very open, and exposes all the inner whorls (PL LII, figs 1, 2, 4 j 

 PL LIIa, figs. 3, 5). 



The aperture is oblong (PI LII, fig. 2), it is widest at the inner side near the return 

 of the spire, and tapers away at the outer side towards the siphonal area. 



The lobe-line resembles the type of that of Ae^. Jmnesoni (PL LII, fig. 3). The 

 siphonal lobe is shorter and wider than the principal lateral. The siphonal saddle forms 

 a regular arch, festooned with simple folioles on the sides, and at the termination. The 

 principal lateral lobe is long and narrow ; it has four lateral digits on each side, and one 

 long terminal bifid process. The lateral saddle, much wider than siphonal, is festooned 

 all round with bilobed leaves, the one side being almost the copy of the opposite. The 

 lateral lobe is smaller than the principal, which it very much resembles in shape and 

 digitations. The auxiliary saddle is nearly as large as the siphonal, which it much 

 resembles in the style of its folioles ; the accessory lobes are small oblique processes. 



I consider the specimen figured in PL LII, figs. 1 and 2, as the best type of this form, 

 as it differs very little from the specimens met with as Am. Jamesoni in most collections of 

 Yorkshire Lias Ammonites. My old friend Mr. Leckenby, F.G.S., often pointed out 

 from a type specimen he had, the points wherein he considered the Yorkshire shell to be 

 difl'erent from the true Am. Jamesoni, Sow., of the Mineral Conchology. 



The specimen figured in PL LIIa, figs. 1 and 2, was collected in Robin Hood's Bay, 

 from the same rock whence the large specimen was obtained ; in this example the 



