392 THE LIAS AMMONITES. 



cipal lateral lobe is as long but not so wide as the siphonal ; its sides have two small 

 lateral digitations and a terminal one divided into two branches. The lateral saddle is 

 deeper and narrower than the siphonal, and terminates in one short foliole on the outer 

 and a longer and more complicated foliole on the inner side. The lateral lobe is 

 much smaller than the principal lateral, and has three lateral digitations and a longer 

 trifid terminal one. The auxiliary lobe terminates in two folioles, and the other auxiliary 

 lobes and saddles are small, insignificant processes. A comparison between the lobe- 

 iine of this shell and that of Amal. oxynotus figured on the same plate (fig. 5) will enable 

 the student to realise the difi'erence between the two lobe-lines of these two representative 

 species, which have been hitherto confused together as one and the same species. 



Affinities and Differetices. — The more regular convex form of the shell, the regularity 

 of the line of flexure of the folds, the difi'erence in the arrangement of the lobe-line, the 

 occlusion of the umbilicus, and extreme involution of the spire, form a group of diagnostic 

 characters by which this Ammonite may be easily distinguished from A. oxynotus. 



Locality and Stratigr a pineal Position. — I only know this species from the pyritic 

 Marl Bed near Black Venn, between Lyme and Charmouth, in the zone of Amal. oxynotus. 

 On some of the fossils I have observed young shells of Aegoceras densinodum adhering to 

 their surface. Eor the other forms in the Amal. oxynotus Bed I must refer to pp. 52 — 55 

 of this work. 



Amaltheus Simpsoni, Bean. PI. XLVII, figs. 4 — 7. 



Ammonites Simpsoni, Bean, MS. Simpson's Monograph on Ammonites of the 



Yorkshire Lias, p. 37, 1843. 

 — — Simpson. Fossils of the Yorkshire Lias, p. 79, 1855. 



Amaltheus — Tate and Blake. Yorkshire Lias, p. 291, pi. viii, fig. 4, 1876. 



Diagnosis. — Shell discoidal, very much compressed ; whorls four or five, extremely 

 involute, the inner seven eighths concealed ; outer volution one half the diameter of the 

 shell, and inflated and rounded near the spiral suture ; ribs nearly obsolete ; sides with 

 twenty undulating folds limited to the inner two thirds, outer third with numerous fine 

 striae bent forward toward the aperture; the keel thin, sharp, and cutting; aperture 

 acutely lanceolate. 



Dimensions. — Transverse diameter 100 millimetres; height of the outer whorl at 

 aperture 50 milUmetres ; transverse diameter of the small shell (fig. 6) 40 millimetres ; 

 height of the aperture 20 millimetres. 



Description. — The only example of this shell I have seen is the one figured, 

 which now belongs to the Woodwardian Museum, and was formerly the property of my 

 old friend Mr. John Leckenby, F.G.S., in whose collection I studied it. This shell is dis- 

 tinguished from Amal. oxynotus by the inflation of the inner third of the whorls, the lesser 



