446 THE LIAS AMMONITES. 



Harpoceras OVAT0M, Toung and Bird. PL LXIII, figs, 4, 5, 6, 7. 



Ammonites OVATUS, Young and Bird. Geological Survey, p. 251, pi. xiii, fig. 4, 



1822. 



— — Phillips. Geology of Yorkshire, pi. xiii, fig. 10, 1829. 



— — Simpson. Monogr. of Lias Ammonites, p. 35, 1843. 



— — — Fossils of the Yorksh. Lias, p. 76, 1855. 

 Hakpoceeas CECILIA, Ueiwec^e. Tate and Blake's Yorksh. Lias, p. 305, pi. ii, 



fig. 6, 1876. 



Diagnosis. — Shell discoidal, compressed, and carinated ; whorls ovoid, one half invo- 

 lute ; radii sigmoidal, bifid, each feebly-developed stem divides into two branches, of which 

 from fifty to sixty are found on the convex side of one volution ; the inner margin slopes 

 evenly down to the penultimate whorl without any spiral angle ; keel sharp, prominent, 

 and entire, having two narrow shallow grooves on each side thereof. Aperture ovate, 

 sides with convex lateral lappets, and a short projecting abdominal spine. 



Dimensions. — Transverse diameter 105 millimetres; height of the last whorl 40 

 millimetres ; width of umbilicus 29 millimetres ; height of aperture 28 millimetres ; width 

 23 millimetres. 



Description. — This is a well-marked form in the Upper-Lias beds near Whitby ; it 

 often occurs as a rough, irregular-formed, discoidal, compressed shell, having ovoid whorls, 

 which are one half involute ; the inner third of the whorls slopes gently down to the spire 

 without any angular margin such as is seen so well marked in Harp, exaratum, Harp. 

 Lythense, and Harp, elegans ; the sides of the whorls are, in fact, nearly convex in a regular 

 well-formed shell, sometimes, however, the inner third of the whorl protrudes beyond the 

 convex curve ; the ribs arise by a number of feeble striae from the inner margin, which 

 soon coalesce and form stems, and each soon divides into two branches ; the ribs are all 

 twice bent, first a little forward, then well bent backward, and, lastly, again well directed 

 forward; the radii are not prominent (PI. LXIII, fig. 4). 



The shell slopes away to the siphonal area, which is the narrowest portion of the 

 volution. The keel is sharp, narrow, entire, and prominent, with two small longitudinal 

 sulci on each side of it, on which the forward directed portion of the ribs finally 

 vanish (Pi. LXIII, fig. 6). 



The aperture is oval, and the walls form two lateral convex lappets and a forward- 

 directed abdominal spine ; just before the aperture the walls of the shell become slightly 

 expanded, and show irregular foldings (PI. LXIII, figs. 4, 5). 



The suture line is of the radians type. The siphonal lobe is large, with two widely- 

 diverging branches, and is nearly the size of the siphonal saddle, which is provided with 

 an accessory lobule that divides the space into two unequal folioles, the inner one being 

 the largest. The principal lateral lobe is very large ; it has three lateral denticles on 

 each side, and a long terminal branch ; the lateral saddle likewise ends in two folioles, 



