VESTINAUTILUS CARINIFERUS. 83 



the obscure ridges just described in the adult shell. These prominent ridges 

 probably aided materially in giving firmness to the young shell, which as it 

 became stronger absorbed them (PI. XXIII, fig. 3). 



The earlier whorls (about three of them) are distinctly inflated on the 

 umbilical declivities, but as the shell advances towards the adult stage they 

 become flattened and even concave in the lower half near the aperture. 



The shell increases somewhat rapidly in diameter, the great lateral expansion 

 giving rise to the deep, broadly funnel-shaped umbilicus.^ The body-chamber 

 occupies about half a volution; a little above its base are seen, in well-preserved 

 casts, the impressions, consisting of pittings and other rugosities appertaining to 

 the shell-muscle. These are admirably shown in a specimen from Cork in the 

 collection of the British Museum (Natural History). They are figured in the 

 ' Catalogue of the Fossil Cephalopoda,' British Museum, pt. ii, 1891, p. 106, 

 fig. 15, where a full description of these remarkable impressions will be found. 

 Other marks (Runzelschicht) originating in the soft parts of the animal are the 

 singular rows of minute punctures, not unfrequently met with on casts of the 

 body-chamber ; these have a linear arrangement conforming precisely with the 

 outline of the aperture, and are remarkably regular. The surface of the test is 

 perfectly smooth, only fine strise of growth being seen when it is suitably 

 preserved.^ 



The septa are very numerous ; upon the umbilical declivities the sutures are 

 arched slightly backwards, and form in crossing the periphery a shallow, back- 

 wardly directed sinus ; they increase slowly in their distance apart. Where the 

 periphery measures 30 mm. in width they are 5 mm. apart, and this distance is 

 increased to 8 mm. where the periphery is 45 mm. wide (PI. XXIII, fig. 2). 



The siphuncle is situated a little above the centre of the septum in the adult 

 shell, but its position probably varies somewhat in the course of shell-growth. 



The surface of the specimens that I have examined, and they are many, exhibit 

 no markings except what may be described as lines of growth ; but, as I have 

 remarked in the description of V. paucicarinatus, this negative evidence must not 

 be taken as conclusive, because mineral action in these fossils, though leaving a 

 smooth and apparently uninjured surface, may have, as in the instance referred 

 to, completely destroyed all surface markings not coarse enough to withstand it. 



^ The gigantic individual, whose dimensions are given on the next page, was presented to the 

 Museum of Science and Art, Dublin, by Mr. Thomas Plunkett, ^M.E.I.A., of Enniskillen, an ardent 

 geologist and archaeologist. 



2 A specimen, apparently belonging to this species, from Glenbane, county of Limerick, shows 

 two strong and prominent folds near the aperture on each side. Through an oversight, this specimen 

 has not been figured in its proper place. It is figured in PI. XXVIII. 



