196 PALEONTOLOGY OF KENTUCKY. 



there is a large part of a volution missing can not be doubted. Thus it 

 appears that the illustrated specimen, in its perfect condition, had six full 

 volutions. Prof. Hall named this species in honor of Prof. O. C. Marsh, the 

 eminent geologist. 



Formation and Locality. Found in tho N iagara rooks of the quarries east of the city of Louis- 

 ville, where fragments of this shell are not rare, but fair specimens are not often found. 



NaUtilUS. Breymus. 



Etymology : nautilos, a sailor or navigator. 



Nautilus maximus. CONBAD. 



Plate XXIV., figure 1. 



Cyrtoceras maximum, Conrad. Geol. Survey of N. Y., Pal. Dept. First Ann. Rep. 1838. 

 Nautilus maximus, Hall. Illust. Dev. Fossils 1876. 

 Nautilus maximus, Hall. Pal. N. Y., Vol 6, pt. 2, p. 4181876. 



Shell very large, sub-discoid, gibbous, becoming very ventricose. Volutions 

 about three, contiguous, not re-entrant. Umbilicus wide and deep, exposing 

 all the volutions. Transverse section sub circular, flattened on the concave 

 dorsal side ; tube regularly and gradually enlarging to a point near the aper- 

 ture; apical angle about fourteen degrees. The body -whorl, or chamber of 

 habitation, very large and ventricose, occupying half a volution and more. In 

 the larger individuals it is free from the inner volution for about one-third its 

 length. 



Length of the grand chamber more than twice its greatest diameter, gradu- 

 ally contracting towards the aperture from a point about two-thirds of its 

 entire length from the base. Aperture slightly oblique to the axis of the tube, 

 opening upward. Air-chambers numerous, regular and very deep, gradually 

 increasing from the apex, and measuring on the convex side sometimes more 

 than thirty millimeters ; the last one shallower than several of those pre- 

 ceding. 



Septa regular, distant, very concave, the concavity greater than the depth 

 of the air-chambers ; strongly imbricating. The distance between the septa is 

 variable in different specimens, but in the same individual is nearly constant, 

 with a gradually increasing distance. Siphuncle large, sub-central, slightly 

 expanding between the septa. 



The ventral side of the shell is indicated by a sharp, narrow, longitudinal 

 raised line on the outer face of the chambers, on the convex side of the volu- 

 tions. Test seldom preserved. 



