18 SYNOPSIS OF THE CHARACTERS OF THE 



(and inner wliorls of the old) marked witli strong longitudinal (spiral) sulci, crossed by very fine striae ; last volu- 

 tion of adult specimens marked with coarse transverse sigmoidal stria3 only. 



I have named this species from the remarkable difference in striation between the young and old volutions, 

 which is so great they might be taken for different species ; it resembles the N. subsnlcatus, Phil., most nearly, 

 but is distinguished by the character of the striation, and by not having, at any period of growth, either 

 the two keels on the back, the keel on the sides, or any concavity on those latter, the sides being regularly con- 

 vex, and the back broader in proportion to the width of the sides. Diameter, three inches one line ; length of 

 mouth, one inch one line ; width, eleven lines. It grows much larger. 



Nautilus (Discites) latidorsatus. M'Coy. (PL IV. fig. 16). 



Sp. Ch. — Discoid compressed, smooth ; volutions entirely exposed in a very wide, shallow, undefined um- 

 bilicus ; whorls cordate, triangidar ; sides margined exteriorly by a prominent rim or keel, within which is a 

 shallow sulcus, from whence into the umbilicus the sides are evenly convex ; back very broad, flat, nearly 

 equalling the side in width. 



This species differs, from all of the genus that I know, in the cordate or triangular section of the whorls, 

 and in the prominent margin to the sides, formed, as it were, by the projection of the flat back. There are very 

 faint traces of spiral stri» on the inner, or yoimg volutions ; septa simple, concave outwardly. Diameter, one 

 inch seven lines ; length of mouth, six lines ; width of back, five lines. 



Nautilus (Discites) mutabilis. M'Coy. (PI. III. fig. 7)- 



Sp. Ch. — Discoid, very much compressed ; whorls five or six, entirely exposed, quadrangular, aperture sa- 

 gitate ; back very concave, less than oue-fomth the width of the side, umbilical slope as broad as the back, 

 steep, concave, with an angular margiji ; two outer whorls in adult specimens perfectly smooth, inner ones 

 marked with strong equal spiral sulci. 



The change of character of the surface from the inner to the outer whorls is even greater than in the 

 Nmitihis (Discites) discors, M'Coy, the inner volutions being spirally striated as in that species, but the outer 

 turns being perfectly smooth ; it differs from that species, besides the smoothness of the outer whorls, in the 

 much more compressed form ; long, narrow mouth, and the narrow and very concave back ; from the A". (Dis- 

 cites) trochlea, M'Coy, it differe in the striation of the inner whorls. Diameter four inches four lines ; width of 

 side, exclusive of marginal slope, one inch two lines; width of back three lines, greatest width of mouth at the 

 umbiUcal angles seven lines. 



Nautilus (Discites) oxystomus. Phil. sp. 



Nautilus oxystomus. Phil. Geol. York. 



Sp. Ch. — Discoid, compressed, smooth, back acute ; volutions half exposed; sides of the inner whorls with 

 two sharp spiral ridges. 



Several fragments of this remarkable species have occurred to me, but none of them shewed more of the 

 septa than has been abeady indicated by Professor Pliilhps ; the length of the aperture is equal to half the 

 diameter of the shell. 



Nautilus (Discites) planoteegatus. 'M'Coy. (PI. II. fig. 2). 



Sp. Ch. — Discoid, compressed, whorls quadrangular ; sides flat, smooth; back flat, smooth, three-fourths the 

 width of the sides ; inner whorls exposed, about five in number ; septa simple, about half an inch apart. 



This fine species often reaches to ten inches in diameter ; it is distinguished from the N. complanatus, 



