CARBONIFEROUS FOSSILS OF IRELAND. 19 



Sow., by its very broad, flat back, and simple septa ; from the N. discus, Sow., by tlie broad, flat back, and 

 the greater spaces between the chambers ; the N. quadratus, Flem. is distinguished by its striated surface. 

 Diameter, four inches ; width of last whorl, one and a-lialf inches ; breadth of back, one inch three lines. 



Nautilus (Discites) subsulcatus. Pliil. sp. 



Nautilus subsulcatus. Phil. Gc-ol. York — Nautilus subsulcatus. Sow. GeoL Trans. 



Sp. Ch. — Whorls about five, entirely exposed ; back narrow, mesial portion concave in the young, flatter 

 in adult specimens, separated from the lateral portion of the back by a ridge on each side ; lateral portions of the 

 back flat, oblique, narrower than the mesial portion ; sides divided nearly in the middle by a spiral ridee ; the 

 outer portion of each side being concave, the inner slightly convex ; surface sigmoidally striated ; siphuncle 

 dorsal. Diameter three and a half inches, tliickness eleven lines. 



Nautilus (Discites) sulcatus. Sow. sp. 



Nautilus sulcatus. Sovs. Min. Con Nautilus sulcatus. Phil. Geol. York. 



Sp. Ch. — Whorls oblong ; gi-eatest thickness at the edge of the umbilicus ; three keels on each side, exclu- 

 sive of the raised edge of the umbiKcus, or of the latero-dorsal angle, including between them four grooves, 

 two large next the back, and two small next the umbilicus ; siphuncle nearly dorsal ; surface with sharp, sig- 

 moidal strice. 



The great nimibcr 'of longitudinal grooves which furrow the back and sides of this species, give it a very 

 strongly marked character ; there are five keels on each side, one forming the angle which bounds the back ; at 

 a little distance from this, on the side, there is another lai'ger, the two being separated by a shallow, concave 

 space ; there is then a concave space exceeding in width the last two keels ; from this to the edge of the umbi- 

 licus there are three smaU keels separated by two small furrows ; the back is concave ; the whorls axe nearly all 

 exposed ; the surface has fine sigmoidal stria2. Diameter one inch seven lines ; thickness seven lines ; length of 

 mouth eight lines. 



Nautilus (Discites) tetragonus. Phil. sp. 



Nautilus tetragonus. Phil. Geol. York. 



Sp. Ch. — Discoid, compressed; whorls quadi-angular, back flat or slightly concave, with a small longitudi- 

 nal ridge on each side ; umbilicus very large, cii'cular, sides with sharp curved strise, forming thin, prominent 

 plaits at the edges. 



This shell is very local in its distribution ; it varies considerably in its character, according to the age and 

 state of conservation of the specimen. Diameter about one inch. 



Nautilus (Discites) trochlea. M'Coy. (PL III fig. 4). 



Sp.Ck. — Discoid, compressed, smooth, volutions exposed, sides broad, slightly convex; back very concave, 

 narrower than the inner edge of the volution, considerably less than one-half the width of the sides ; margin of 

 the septa very concave outwardly on the sides and back, produced into a very acute angle on the latero-dorsal 

 ridges. 



This species may be distinguished from the N. planoterqatiis, M'Coy, by the deep concavity of the back, 

 which is also much naiTower than in that species, the umbilicus is also smaller. The large size of the 

 umbilicus distinguishes it from the N. planidorsatus, Portk. Thickness of last whorl (breadth of side) two 

 inches four lines, width of back eleven lines. 



