432 THE LIAS AMMONITES. 



E. Cross, F.G.S., and Peter Cullen, from a shaly ferruginous pyritic bed of clay in the 

 north corner of Robin Hood's Bay, Yorkshire Coast, referred by them to the Aeg. 

 Jamesoni-7.on& of the Middle Lias, and containing other fossils of this zone. 



Harpoceras nitescens, Younff and Bird. Pi. XLIX, figs. 2 — 7. 



Ammonites NiTESCENs, Toung and Bird. Geol. Surv.York. Coast, p. 257, 1828. 



— — Simpson. Monogr. Ammon., York. Lias, p. 45, 1843. 



— RADIANS AMALTHEi, Oppel. Mittl. Lias Schwabens, p. 5 l,tab. 3, fig. 1, 1853. 



— NITESCENS, Simpson. Fossils of Yorksh. Lias, p. 87, 1855. 



— NoKMANiANUS, Oppel. JuraformatioD, p. 168 (non d'Orb.), 1856. 



— FALCiFEK, Quenstedt. Der Jura, p. 173, tab. 22, fig. 28, 1856. 



— Algovianus, Oppel. Palaeontol. Mittheilung, p. 137, 1862. 



— — Regnes. Geol. et Paleontol. Aveyron., pi. ii, fig. 1, 



1868. 

 Harpoceras Algovianum, Tate and Blake. Yorkshire Lias, p. 302, pi. viii, 



fig. 1, 1876. 



Diagnosis. — Shell discoidal, compressed, carinated, and slightly involute ; volutions 

 six, inner very little concealed ; sides of vi'horls flat, vrith from twenty-five to thirty 

 straight, sharp ribs, developing a series of blunt tubercles near the siphonal area, and 

 then bending forward to the keel, which is thick and prominent without lateral sulci ; 

 aperture oblong, with straight sides ; suture-line very complicated. 



Dimensions. — A prolonged search in the Whitby Museum during the summer of 

 1881, with Mr. Simpson, among some of the unfigured species of Lias Ammonites 

 recorded in bis list, afforded me an important insight into the characters of several of 

 his species, and I had the pleasure of finding in the type specimen of Am. nitescens an 

 unmistakable example of the Am. Algovianus, Oppel. I was anxious to have figured 

 Young's type shell, but Mr. Simpson will not allow any specimen to leave the building ; 

 afterwards I had the good fortune to find a specimen of this shell in my friend Mr. 

 Slatter's cabinet, collected from the Amal. margaritatus-heA. in the Midland Counties, 

 and figured in PI. XLIX, figs. 2, 6, and 7. Much better examples of this species are 

 found in the Marlstone near Chipping-Norton, where it occurs in a brown sandy bed, 

 which has well preserved the shell-texture. I am indebted to Mr. James Windus of that 

 town for the small specimen I have figured (3, 4, and 5), and which I have magnified 

 two diameters, as the fossil was too small to exhibit its characters without enlargement. 



The shell is compressed and carinated. 'Ihe sides of the whorls are flat, and ornamented 

 with straight, simple, angular, well spaced-out ribs, which develop a series of obtuse 

 tubercles at the side of the area before they bend suddenly forward towards the aperture. 

 In the centre of the siphonal area is a high, prominent, sharp keel, which conceals and 

 overlaps the siphuncle that lies beneath, and this structure is very well shown in 



