270 THE LIAS AMMONITES. 



Diagnosis. — Shell discoidal, depressed ; whorls slightly involute ; sides convex and 

 largely costated j ribs thick, bent, remote, twenty-four in a whorl ; back flat, strongly 

 carinated, with a deep furrow on each side of the keel ; aperture quadrato-hemispherical. 



Dimensions. — Transverse diameter of figured specimen, 230 millimetres ; height of the 

 last whorl at the aperture 65 millimetres; width 75 millimetres; whorls about one fifth 

 involute. Many of the large specimens from Keynsham measure 18 inches in diameter. 



Description. — This Ammonite attains a gigantic size, it has a flattened discoidal shell, 

 formed of six or seven slightly involute whorls, having the inner ones largely exposed ; 

 their sides are convex, sub-cylindrical, and separated by a deep suture, having strong 

 prominent ribs, from twenty-four to thirty-two in a whorl, which are sharp, distant, 

 simple, arched, and terminate abruptly near the border where they are interrupted ; the 

 siphonal area forms a broad flattened arch, and the middle line is occupied by a strong 

 thick keel, bounded on each side by a deep furrow ; the aperture is rounded or sub- 

 quadrangular. 



The septa are symmetrical, divided on each side into three lobes and three saddles 

 formed of unequal parts. The siphonal lobe is narrow, one third longer, and a little 

 wider than the principal-lateral lobe, and has on each side five nearly equal-sized serrated 

 digitations (PI. I, fig. 3). 



The siphonal saddle, one fourth wider than the principal-lateral lobe, is divided into 

 seven leaves of unequal size. The principal-lateral lobe, a little longer than wide, has on 

 each side three elongated foliated digitations. The lateral saddle, twice the size of the 

 principal lateral lobe, is formed of eight or nine subdivided lobules. The infero-lateral lobe, 

 smaller than the principal-lateral, has four external and three internal digitations. The 

 auxiliary saddle is small, with five small lobules. The auxiliary lobe is small, with three 

 digitations. 



Affinities and Differences. — This species was first described by M. Bruguiere in the 

 ' Encyclopedic Methodique,' torn, i, p. 39, under the name Ammonites bisulcata. The 

 diagnosis included two forms — (a) Ammonites Bucklandi, Sow. ; and (b) Ammonites 

 multicostatus, Sow. 



(a) " Ammonites costis simplicibus raris, dorso bisulcato carina acuta intermedia." Brug. 



(b) " Ammonia cornu spina in ambitu eminent e, striis lateralibus ex toto orb em 

 valde extimum trajicentibus." Lister. 



The variety a had been previously figured by Lang in his ' Historia Lapidum 

 figuratorum Helvetia?/ p. 95, tab. xxiv, fig. l,and described as "Ammonia cornu striatum 

 valde striis inteyris in spinam inter duoa aulcos eminentem abeunlibua." 



I propose to retain Sowerby's name for var. a, and Bruguiere's name, bisulcata, 

 for var. B, as the forms are very distinct and readily distinguished from each other. 

 A. multicostatus, Sow., will then be a synonym of A. bisulcatus, Brug. 



Arietites Bucklandi has twenty-four to thirty-two ribs, which gradually disappear on 

 the outer border, without tubercles near the angle of the sulci. 



