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. 2c). 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 imterior 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 Sypho, 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 Nawt. xic-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. Wetherell, who have afforded me 
the use of their specimens, I have before me a series of shells from Sheppy and the 
