54 EOCENE MOLLUSCA. 



the present remains as forming part of Munster's Clymenia, a genus distinguished 

 by its having the whorls exposed. 



The Aturia zic-zac is a smooth, involute shell, more or less ventricose or depressed ; 



the septa are outwardly deeply concave; and, owing to the regular curve in which 



the dorsal lobes are reflected towards the axis of the shell, they present, when viewed 



sideways, some resemblance to the letter S ; the lateral lobes are more or less narrow, 



and taper rather suddenly towards the inferior extremity, which extends nearly to the 



preceding septum ; but they are without the sinus which characterises the lateral lobes 



of Naut. Parkinsoni. The English shells are generally either casts in, or filled with 



pyrites, and it is difficult to ascertain the character of the siphuncle from them ; but 



in the Dax shells, in which the calcareous siphon is frequently well displayed, it 



presents a structure widely different from that of the Nautilus. The dorso-marginal 



part of the septum, as I have before observed, is prolonged backwards in the form of 



a wide trumpet-mouthed funnel, which extends not only into the mouth of the funnel 



formed by the preceding septum, but along the preceding siphuncle almost to the 



floor of the third preceding chamber (see Tab. IX, fig. 2a). The calcareous siphuncle, 



therefore, is, in fact, a continuous tube of considerable thickness, composed of portions 



of two distinct tubes ; and within this is contained a soft, friable, calcareous sheath, 



which commences near the extremity of the funnel, where it touches the preceding 



funnel, and extends to the end of the preceding funnel, to the interior surface of 



which it forms a sheath. Although, owing to the thickness of the walls and the 



presence of the calcareous sheath, the actual tube within which the membranous 



siphuncle was contained is not so capacious as might be expected from its external 



appearance, it is yet considerably larger than that of any of the tertiary Nautili ; and 



indeed it is of such size and importance as fully to justify the name Syplto, which Grateloup 



has given to the Dax shell. The siphuncle in the English specimens, so far as its 



character can be ascertained, appears to correspond exactly with that of the Dax 



shells. 



Great diversity of opinion has existed, and, in fact, still exists, as to the identity of 

 the Dax shell with the Naut. zic-zac of Sowerby. The differences which have been 

 relied on for the separation of the two appear to me to result from the more compressed 

 form of the Dax shells ; the specimen figured by Mr. Sowerby, although described as 

 " flattish," being ventricose, and the outline of one of the septa drawn below the shell 

 conveying the idea of greater fulness than in fact characterises the fossil. M. Deshayes, 

 who compared the Dax shells with specimens as well from the Paris basin as from 

 Belgian and English localities, expressed an opinion that the differences were sufficient 

 to form, if not two species, at least two well-marked varieties. I have not myself had 

 an opportunity of examining any French or Belgian specimens ; but, through the 

 kindness of Mr. Sowerby, Mr. Bowerbank, and Mr. Wethcrell, who have afforded me 

 the use of their specimens, I have before me a series of shells from Shcppy and the 



