173 



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 dorsal lobe is naiTow, one- 

 third longer, and a little wider than the supero-lateral lobe, and has on 

 each side, five nearly equal sized serrated digitations. (PL 1, fig. 2, a.) 



The dorsal saddle^ one-fourth wider than the supero-lateral lobe, 

 is divided into seven folioles of unequal sizes. The supero lateral 

 lobe, a little longer than wide, and has on each side three elongated 

 foliated digitations. The lateral saddle, twice the size of the supero- 

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

 infero-lateral lobe, smaller than the supero-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 Encyclopedie Methodique, tom. 1, p. 39, under the 

 name Ammonites bisulcata, the diagnosis includes two forms (a) Ammonites 

 Bibcklandi, Sow.; and (b) Amm,onites multicosiata, Sow. 



(a) "Ammonites costis simplidbus raris, dorso bisulcato carina acuta 

 intermedia." 



(b) "Ammonis cornu spina in ambitu eminente, striis, lateralibus ex 

 toto orbem, extimum trajicentibtts." 



The Tariety a had been previously figxired by Lang in his Historia 

 Lapidum Figuratorum Helvetice, tab. 24, fig. 1, and described as 

 "Ammx)nis cornu" striatum striis integris in spinam inter duos sulcos 

 eminentem abeuntibus. 



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

 bisulcata, for var. b, as[the foims are very distinct and readily distinguished 

 from each other. A. multicostata. Sow., will then be a synonym of 

 A. bisulcata, Brag. 



Ammonites Bucklandi has 24 ribs, which gradually disappear on the 

 dorsal border, and there are no tubercles near the angle of the sulci. 



Ammonites bisulcata has a quadrate angular form, with from 34 to 36 

 costae, with a tubercle on each rib near the angle of the dorsal border, 

 from whence the ribs ciu-ve forward into the sulcus. 



There is no other form in the Lower Lias, for which A. Bucklandi 

 can be mistaken. 



Locality and Stratigraphical position. — Ammonites Bucklandi 

 chai-acterizes an important zone of life in the English Lias, the strati- 

 graphical relations of which are best exposed in the magnificent coast 

 section, near Lyme Regis. I have collected this Ammonite likewise 



