20 SYNOPSIS OF THE CHARACTERS OF THE 



TEMN0CHEILUS^ M'Coy. 



Gen. CA.— Shell discoid, involute, umbilicate ; a deep sinus in the middle of the outer lip; septa simple, 

 sif)huncle central. 



This subgenus will comprehend the greater number of the Nmdili of the carboniferous series. From the 

 true genus Nautilus it is externally distinguished by its discoid, flattened form, and the greater number of its 

 volutions, all of which are exposed in a wide, conical umbilicus ; one of the most important structural differences 

 is, the deep notch, or sinus, to be found in the outer lip of the present group, from which it derives its name. 



Nautilus (Temnocheilus) biangulatus. Sotv. sp. 



Nautilus biangulatus. Sow. Min. Con Nautilus biangulatus. P/iil. GeoL York. 



Sp. Ch. — Discoid ; volutions half exposed by a large, conical umbilicus, bounded by a raised margin or 

 keel ; back evenly convex ; septa very convex ; siphuncle central. 



This species is remarkable for the sharp, prominent keel which surrounds the umbilicus ; between 

 the prominent angle on each side, the back is slightly convex, but without any other keels or furrows ; this 

 forms the chief external difference between this species and the T. cariniferus. The mouth is about twice as 

 wide as long ; the septa very concave outwardly ; siphuncle central, or nearly so ; surface smooth. Diameter 

 four inches, thickness two and a half inches. 



Nautilus (Temnocheilus) bisteialis. Phil. sp. 



Nautilus bistrialis. Phil. Geol. York. 



Sp. Ch. — Orbicular, compressed, smooth; back broad, rounded; umbilicus very large, circular, with two 

 strong spiral grooves within the edge. 



This species occurs very rarely, and generally without the external shell ; the sculptured edge of the um- 

 bilicus is a very curious peculiarity. 



Nautilus (Temnocheilus) cariniferus. Sow. sp. 



Nautilus cariniferus. Sow. Min. Con. — Nautilus carinatus et cariniferus. Phil. Geol. York. 



Sp. Ch. — Discoid, vokxtions subpentagonal, half exposed by a wide conical umbilicus, which is margined 

 by an acute keel or ridge, between which and the back there is on each side an obtuse ridge ; siphimcle mar- 

 ginal. 



This species differs from the T. biangulatus principally in having on each side an obtuse keel on the 

 back ; the space between the obtuse keel and the edge of the umbilicus is concave, while that between the one 

 keel and the other is convex ; the general form is that of the N. biangulatus, which it resembles in many 

 respects ; the two keels on the back are sharp and distinct in the young shells, but become more obtuse as they 

 advance in age, and in very old specimens they are sometimes quite effaced, and are then scarcely distinguish- 

 able from the N. biangulatus ; the mouth is twice as wide as long ; the siphuncle is nearly dorsal. Diameter 

 three and a half inches ; thickness two inches two lines. 



Nautilus (Temnocheilus) coronatus. M'Coy. (PL IV. fig. 15). 



Sp. Ch. — Discoid, subglobose ; inner whorls half exposed in a large, steep iimbilicu.s, the acute edge of 

 which is crowned with large, compressed, blunt tubercles ; back flattened ; surface smooth ; septa simple ; si- 

 phuncle nearly central. 



* Tifivu, seco; and x^^^'^t labrum. 



