CCELONAUTILUS GRADUS. 57 



From G. r/ra-dus the present species is easily distinguished by its more rapid 

 rate of increase, and by the absence of the strong ridges ornamenting the 

 peripheral area in the adult of that species (PI. XIX, figs. 3 a, I). The two 

 species are, however, clearly related. 



Bemarls. — The four specimens representing this species were all collected by 

 me in a small mass of decomposing limestone in the railway cutting of the 

 Limerick and Foynes Eailway, near Rathkeale. The same fragment also con- 

 tained many specimens of the young of 0. gradus, A. H. Foord, an Orfhoceras 

 (0. salvum, L. G. de Kon.'), a small Gasteropod, and a species of Goniatite 

 {Pericyhis Dooliylensis, A. H. Foord and G. 0. Crick "), all admirably preserved 

 and showing their finest ornamentation. 



Locality. — Doohyle, county of Limerick. 



e. CffiLONAU'i'iLTJS GHADUS, A. H. Foovd. Plate XX, figs. 1 — 9. 



1891. C(ELONATJTiLUS GEADUS, A. H. Foord. Cat. Fos.«. Ceph. Britisli Museum, 



pt. 2, p. 126, fig. 19. 



1893. IStbobockeas ouadtjs, A. Hyatt. Carboniferous Cephalopods. Secoud 



paper. Geological Survey of Texas, Fourth 

 Annual Eeport, 1892, p. 411. 



Description. — Shell thick-discoid, composed of two and a half or nearly three 

 whorls, all exposed in a deep, step-shaped umbilicus, having a very large central 

 vacuity. Section of the whorls distinctly quadrate and tetragonal. The peri- 

 pheral area is broad and slightly depressed along the median zone, with two distinct 

 keels on each side of it, one on the outer or lateral edge, the other just inside of 

 this. Bordering the inner keel on each side there is a slight prominence, the space 

 between these prominences forming a shallow depression. The sides are con- 

 siderably narrower than the peripheral area; thus, where the latter is 18 mm. 

 across, the former measure only 12 mm. In the young shell this discrepancy 

 goes still further; thus, where the sides of the whorl are 5 mm. in width, the 

 peripheral area is 9 mm. across. The sides are slightly convex, and slope out- 

 wards towards the umbilical margin (PI. XX, fig. 4). They bear on the inner 

 whorls faint transverse folds with dimple-like depressions between them ; these 

 become obsolete as the shell attains the adult stage. The body-chamber occupies 

 about half a volution. Towards the aperture the peripheral angles become 

 rounded off both in the young and in the adult shell; in the latter, distinct thick 

 folds, forming tubercular elevations, may be developed on the peripheral border 



^ ' Calc. Carb. Belgique,' pt. ii, p. G7, pi. xli, fig. 6. 



2 . Cat. Foss. Cepb. Brit. Mus.,' pt. 3, 1893, p. 151, fig. 72. 



10 



