276 THE LIAS AMMONITES. 



Dimensions. — Transverse diameter of specimen figured on PL III, fig. 1, 200 milli- 

 metres ; height of aperture 70 millimetres ; width of aperture G4 millimetres. 



Description. — The table of synonyms prefixed to this article is very limited when com- 

 pared with the lists given in the works of Professors d'Orbigny and von Hauer ; it is 

 due, therefore, to the reader that I should give my reasons for excluding many of 

 the citations of my contemporaries. In the description of Arietites Bucklandi, Sow., 1 at 

 p. 270, I have stated that M. Bruguiere described, under the name of Ammonites 

 bisulcata, two distinct forms — (a) Ammonites Bucklandi, Sow., and (b) Ammonites multi- 

 costatus, Sow. 



The variety a had been previously figured by Martin Lister in his ' Conchyl. Angliae,' 

 p. 207, tab. 6, fig. 3 ; and by Lang in his ' Iiistoria Lapidum Figuratorum Helvetiae,' 

 tab. 24, fig. 1. The variety b was included in Bruguiere's general diagnosis of Ammo- 

 nites bisulcata, and most authors have followed Bruguiere in grouping both forms under 

 one specific name. I am inclined, however, to think that Sowerby was right when he 

 figured the two forms under distinct names. M. d'Orbigny and Professor von Hauer, 

 in their works already cited, have figured good type forms of A. multicostatus, Sow. I 

 assume that this is the recognised type of Bruguiere's A. bisulcata in Prance and Germany, 

 whereas the true A. Bucklandi, Sow., is seldom figured in any work on Lower-Lias 

 fossils. I have, therefore, determined to retain Bruguiere's name for his var. b, and 

 Sowerby's name for var. a (see p. 270). In accordance with this view I have excluded all 

 references to the synonyms of authors, unless I had a figure or a specimen for my guide, 

 to determine the form referred to in the citation ; this mode of proceeding has necessarily 

 diminished the list, but makes it more correct for reference. 



Arietites bisulcatus has a depressed discoidal shell, with a strongly tricarinated siphonal 

 area and subquadrate whorls. The sides are provided with from thirty to forty simple, 

 narrow, slightly-bent ribs, each terminating in a blunt tubercle, near the siphonal border. 

 Prom the tubercle the rib bends sharply forwards, and disappears at the outer carinae. The 

 area is flat and tricarinated, the median keel a little more prominent than the laterals, and 

 the two sulci, although well marked, are not very deep in the specimens I have examined ; 

 the lateral carinas are well defined ; the shell is bevelled off from the ridge to the row of 

 tubercles, and presents a series of oblique undulations occasioned by the vanishing of the 

 ribs. The spire is composed of subquadrate whorls; in a specimen, 5f inches in 

 diameter, there are six whorls visible ; on the outer whorl are thirty-seven ribs ; on the 

 second, thirty ; on the third, twenty-nine ; on the second, twenty-five ; so that in this 

 example the number of ribs increases with the number of the whorls. The aperture 

 is quadrate, rather higher than wide, a little enlarged at the spire, and sinuous above, 

 where it is marked by the central keel, the sulci, and lateral carinae. (PL III and IV.) 



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

 three saddles formed of unequal parts (PL III, figs. 1, 2, 3 ; PL IV, fig. 4). Siphonal 

 1 'Proceedings of the Cotteswold Naturalists' Club for 18G3,' p. 135. 



