490 UNITED STATES GEOLOGICAL SURVEY OF THE TEltMTOEIES. 



and more or less deeply embracing, or merely in contact; septa concave on 

 the surface, facing the aperture, with their margins nearly straight, arched, ' 

 a little waved, or sometimes flexed into a simple rounded or angular back- 

 ward extension, or lobe, on each lateral surface, or sometimes on the inner, 

 and very rarely also on the outer or peripheral side* ; siphuncle simple, cylin- 

 drical, and most usually central or nearly so, very rarely placed nearly against 

 the peripheral side ; surface smooth, striated, costated, or with revolving 

 carina and furrows, sometimes also ornamented with nodes ; aperture reni- 

 form, sublunate, cordate, subsagittate, or varying from oval to subcircular or 

 quadrate ; lip thin, generally sinuous on the outer side and inner lateral 

 margins, each of which latter is sometimes, though rarely, drawn out into a 

 spout-like projection. 



The foregoing diagnosis and synonymy are drawn up so as to include, 

 along with the typical species, a rather wide range of extinct forms, gener- 

 ally believed by high authorities to belong to this genus. Some of the latter, 

 however, differ so materially from the typical Nautili that they ought to be 

 at least entitled to rank as distinct subgenera. A review of the numerous 

 fossil forms of various ages shows that there are other equally marked but 

 unnamed sections; though only the following will be especially noticed here: 



1. nautilus, Linnaeus (typical). 



Shell subglobose, discoid, or more or less compressed; umbilicus 

 often closed, or comparatively small ; volutions generally deeply 

 embracing and rounded, oval, or rarely subangular on the periphery, 

 or near the umbilicus ; septa merely arcuate, nearly straight, or waved 

 on each side, and sometimes on the periphery ; surface smooth, 

 striated, or with small transverse costas that are arched or divari- 

 cating on the sides and periphery. — (Type as already cited.) 



2. TEMNOCHILU3, McCoy. 



Shell subdiscoid, with a large, generally deep umbilicus, and a 

 broad flattened or moderately convex periphery ; volutions usually 

 little embracing, much widened transversely, and angular, and some- 

 times subnodose around each prominent lateral margin ; septa merely 

 a little arcuate on the margins; surface with only marks of growth. — 

 (Nautilus coronatus, McCoy.) 



* Still more rarely there are two angular lateral lubes on eacb side. 



