444 THE LIAS AMMONITES. 



Harpoceras Lythense, Young and Bird, PI. LXII, figs. 4, 5, and 6. 



Ammonites Lythensis, Young and Bird. Geol. Surv. Yorksh. Coast, p. 267, 1828. 



— — Phillips. Geology of Yorkshire, pi. xiii, fig. 6, 1829. 



— — Simpson. Monogr. of Lias Ammonites, p. 33, 1843. 



— concavus, d'Orbigny. Terr. Jurass., p. 358, pi. 116, 1842. 



— Lythensis, Oppel. Juraformation, p. 24, 1856. 



— — Quenstedt. Der Jura, p. 248, tab. xxxv, fig. 5, 1858. 



— — Dumortier. Depots Jurass., partie iv, p. 56, pi. xi, figs. 9, 



10, 1874. 

 Harpoceeas Lythense, Tate and Blake. Yorksh. Lias, p. 304, pi. ii, fig. 4, 1876. 



Diagnosis. — Shell stout, discoidal, depressed, and carinated ; volutions four or five, 

 nearly concealed ; outer whorl in height exceeding the semi-diameter of the disc, thicker 

 near the inner margin (which is acute, with rectangular walls), and sloping towards the 

 outer margin, which terminates in a thick keel ; inner half of the whorl longitudinally 

 depressed and almost smooth, outer half sloping from a ridge towards the siphonal area, 

 and covered with strong, obtuse, bent radii, which vanish on the side of the area; 

 umbilicus narrow, with deep sides, rising stair-like from a sunken pit-like depression ; 

 aperture high and compressed, acute towards the margin, and deeply grooved by the 

 return of the spire. 



Dimensions. — Transverse diameter 95 millimetres; height of last whorl 50 milli- 

 metres; height of aperture 50 millimetres; breadth 23 millimetres ; width of umbilicus 

 17 millimetres. 



Description. — This is a well-known, and highly-characteristic species of the Upper-Lias 

 beds near Whitby, which has been sadly misrepresented by bad figures. It may truly be 

 said that Mr. Gawan's delineation of Harp. Lythense is the first accurate representation 

 of the Yorkshire type. D'Orbigny, under the false name Am. concavus, figured a French 

 shell obtained from the Upper Lias, which he identified with Sowerby's Am. concavus ; 

 this was a grave mistake, as the Am. concavus, Sowerby, is a very different shell, belonging 

 to the Harp.-Sowerbii zone of the Inferior Oolite. I have no doubt therefore that the 

 subject of d'Orbigny's figure is a true Harp. Lythense, which differs little from the 

 typical form which this species assumes in the Yorkshire beds, and of which I have 

 selected a very fine average specimen for the beautiful figure in PI. LXII, figs. 4 — 6. 



This shell is stout, depressed, discoidal, and carinated; the whorls are deep and 

 extremely involute, the outer whorl exceeding in height the semi-diameter of the disc ; on 

 the inner half of the whorl there is a slightly depressed, longitudinal space, which 

 near the middle rises into a low ridge, whence to the margin the side slopes away 

 to the area, and forms a good character of the species ; the keel is strong, but often 

 absent, as the syphon lies safely buried in the mould of the shell ; the keel is formed by 

 a continuation of the sides, and is separated from them by a slight lateral depression. The 



