HARPOCERAS LYTHENSE. 445 



ribs, from fifty to sixty in number, are sigmoidal in form, feebly developed at the 

 inner half of the whorl, and strongly developed on the external half from the low ridge 

 outwards ; they are here broad, flattish waves of the shell, with undulations of a 

 similar breadth between. The shell-structure is very well shown in one of my specimens, 

 in which the outer and inner laminae are freely exposed ; the outer lamina has rather a 

 coarse texture, whilst the inner lamina is thin and smooth. The inner margin or spiral 

 angle is very acute, and, as Young so well expressed it, " the interior edge of each whorl 

 being rectangular, the central part appears like a small round pit with perpendicular 

 sides." The aperture is elongated and compressed; and in one of my specimens the 

 mouth is preserved, the sides of the aperture have two convex lappets, which correspond 

 to the forward curve of the ribs, and there is an abdominal projection produced by the 

 forward flexion of the costae ; in fact, here, as in all the other congeneric forms, the shape 

 of the walls of the aperture is derived from the form and direction of the radii. 



The suture-line is very complicated and not satisfactorily shown on any of my 

 Yorkshire specimens. Professor d'Orbigny, who figured this structure, says that the 

 siphonal lobe is narrower and shorter than the principal lateral and consists of one 

 branch ; the siphonal saddle is very large and divided unequally into two portions by a 

 long accessory lobe, the external portion is much shorter and smaller than the other ; 

 the principal lateral lobe is large and provided without and within with five branches 

 independent of the terminal branch. The lateral saddle is much narrower than the 

 principal lateral lobe, and is divided into two equal parts ; the other saddles are very 

 irregular, the inner lateral lobe resembles the principal lateral in form, but is not more 

 than one third its size, and the three auxiliary lobes are very small. 



Affinities and Differences. — Professor d'Orbigny has given capital figures of this 

 Ammonite, but committed a mistake in identifying it with Sowerby's species Am. concavus, 

 which is an Inferior-Oolite shell belonging to the zone of Harp. Sowerbii, hence the con- 

 fusion that has followed in the lists of species special to different beds ; Ammonites con- 

 cavus being quoted by some as a leading fossil of the Upper Lias, though it is known to 

 those acquainted with its true form that it is never found out of the zone Harp. Sowerbii of 

 the Inferior Oolite. Harpoceras Lythense is a thicker shell, strongly ribbed on the outer 

 half of the whorl and slightly so on its inner half. Harp, concavum is a much thinner 

 shell, with feeble costae on the outer side of the whorl and nearly a smooth surface within. 

 In Harp. Lythense the umbilicus is encircled by an acute angle, and the whorls are cut into 

 vertical walls, whilst in Am. concavus the umbilicus is a concave cavity formed by a sloping 

 away of the angles of the whorls. Harp. Lythense has a distinct and prominent keel, Am. 

 concavus has no keel, but the siphonal area is bevelled away on each side, like the roof 

 of a house. 



Locality and Stratiyraphical Position. — Harpoceras Lythense is collected at Whitby 

 from the Upper Lias in beds of a shaly clay belonging to the zone of Harpoceras ser- 

 pentinum. It was at one time common, but has now become rare. . 



