452 THE LIAS AMMONITES. 



factorily prove. The shell is discoidal and compressed; the whorls about one third 

 involute, are convex on their sides, and the outer two thirds of the height of each are 

 ornamented with from forty to fifty short, slender, biflexed ribs ; the inner third of the 

 whorl, where it slopes towards the umbilicus is smooth, and from the summit of the 

 slope the ribs commence; sometimes they are single, and sometimes they bifurcate, each 

 stem dividing into two branches. The ribs first bend forward then backward, and again 

 sweep forward towards the area and terminate at the side of a shallow sulcus, from 

 which rises a thin prominent keel ; the umbilicus is wide and the inner whorls 

 are exposed, these have an ornate character as each whorl is smooth internally and 

 ribbed externally. The aperture is either ovate or elliptical according to the amount 

 of compression of the sides; the suture-line is well shown in many of my specimens, 

 the chambers appear to have been large, as the septa are moderately distant 

 from each other, and are composed of three lobes and three saddles with smaller 

 auxiliaries. The siphonal lobe is as wide as but shorter than the principal lateral, and 

 ornamented on each side with three short points and two single terminal branches. 

 The siphonal saddle is one third larger than the principal lateral lobe, and divided into 

 two unequal parts by an accessory lobe which arises from the concave surface, the 

 whole ending in large folioles. The principal lateral lobe is large ; it has two or three 

 points on the sides and three longer terminal processes on the summit, and forms a 

 conspicuous object on the side of the mould as it remains well apart from all the others. 

 The lateral saddle is one third narrower than the principal lateral lobe, and terminates in 

 two long folioles. The inner lateral lobe is about half the size of the principal and has 

 an irregular figure with two or three small lateral points. The auxiliary saddle is wide, 

 shallow, and trilobed, and the auxiliary lobes, two or three in number, are small and pointed, 

 and pass into the ventral lobe, which is entirely concealed by the turns of the spire. 



Affinities and Differences. — Some authors regard this species as the true type of 

 Harp, radians and synonymous with it, I have, however, endeavoured to show the 

 affinities and differences which exist between the two forms that lived together in the 

 same sea abounding with congeneric species ; still Harp, striatulum preserved the characters 

 I have pointed out most persistently through life. And the same observation applies to 

 the forms of this species recognised as Am. Thouarsensis, d'Orbig., and Am. Comensis, von 

 Buch, in the respective French and German formations. 



Locality and Stratiyraphical Position. — I have collected this species from the dark 

 Greyish Marl at Blea Wyke, near Robin Hood's Bay, associated with Harpoceras 

 variabile and Harp, insigne, both characteristic fossils of the Lyt. Jurense zone. I have 

 collected this shell likewise from the Lyt. Jurense Ammonite bed at Frocester Hill, 

 Gloucestershire ; these are the only localities I know in England. Sowerby's type specimen 

 came from Blea Wyke, near Peak Hill, Robin Hood's Bay, Yorkshire, where it was collected 

 by Mr. Crawford, of Scarborough, who sent the Ammonite to Sowerby ; the specimen I 

 have figured (PI. LXXXIV, fig. 4) was collected by myself from the same locality. 



